Gaia (mother Earth in Greek
mythology) is an ESA cornerstone space astrometric mission, part of the
Horizon 2000 Plus long-term scientific program, with the goal to
compile a 3D space catalog of > 1000 million stars, or roughly 1% of
the stars in our home galaxy, the Milky Way. Gaia will monitor each of
its target stars about 70 times to a magnitude of G=20 over a period of
5 years. It will precisely chart their positions, distances, movements,
and changes in brightness. It is expected to discover hundreds of
thousands of new celestial objects, such as extra-solar planets and
brown dwarfs, and observe hundreds of thousands of asteroids within our
own Solar System. The mission will also study about 500,000 distant
quasars and will provide stringent new tests of Albert Einstein’s
General Theory of Relativity. 1)2)3)4)5)

Cataloguing the night sky is an
essential part of astronomy. Before astronomers can investigate a
celestial object, they must know where to find it. Without this
knowledge, astronomers would wander helplessly in what Galileo once
termed a ‘dark labyrinth’.

During the satellite’s
expected lifetime of five years, Gaia will observe each star about 70
times, each time recording its brightness, color and, most importantly,
its position. The precise measurement of a celestial object’s
position is known as astrometry, and since humans first started
studying the sky, astronomers have devoted much of their time to this
art. However, Gaia will do so with extraordinary precision, far beyond
the dreams of those ancient astronomers.

By comparing Gaia’s series of
precise observations, today’s astronomers will soon be able to
make precise measurements of the apparent movement of a star across the
heavens, enabling them to determine its distance and motion through
space. The resulting database will allow astronomers to trace the
history of the Milky Way.

In the course of charting the sky,
Gaia’s highly superior instruments are expected to uncover vast
numbers of previously unknown celestial objects, as well as studying
normal stars. Its expected haul includes asteroids in our Solar System,
icy bodies in the outer Solar System, failed stars, infant stars,
planets around other stars, far-distant stellar explosions, black holes
in the process of feeding and giant black holes at the centers of other
galaxies.

The primary mission objectives are:

• Measure the positions and velocity of approximately one billion stars in our Galaxy

• Determine their brightness, temperature, composition and motion through space

• Create a three-dimensional map of the Galaxy.

Additional discoveries expected:

- hundreds of thousands of asteroids and comets within our Solar System

- seven thousand planets beyond our Solar System

- tens of thousands of ‘failed’ stars, called brown dwarfs

- twenty thousand exploding stars, called supernovae

- hundreds of thousands of distant active galaxies, called quasars.

Gaia objective is to provide a very
accurate dynamical 3D map of our Galaxy by using global astrometry from
space, complemented with multi-color multi-epoch photometric
measurements. The aim is to produce a catalog complete for star
magnitudes up to 20, which corresponds to more than one billion stars
or about 1% of the stars of our Galaxy. The instrument sensitivity is
such that distances beyond 20-100 kiloparsec (kpc) will be covered,
therefore including the Galaxy bulge (8.5 kpc) and spiral arms. The
measurements will not be limited to the Milky Way stars. These include
the structure, dynamics and stellar population of the Magellanic
Clouds, the space motions of Local Group Galaxies and studies of
supernovae, galactic nuclei and quasars, the latter being used for
materializing the inertial frame for Gaia measurements.

Background:
Gaia is ESA's second space mission dedicated to astrometry. It builds
on the legacy of the successful Hipparcos mission (1989-1993). 7)
Like Hipparcos, Gaia's observation strategy is based on detecting
stellar positions in two fields of view separated by a 'basic angle',
which for Gaia is 106.5º. This strategy allows astronomers to
establish a coherent reference frame over the entire sky, yielding
highly accurate measurements of stellar positions.

After a detailed concept and
technology study during 1998–2000, Gaia was selected as a
confirmed mission within ESA’s scientific program in October
2000. It was confirmed by ESA’s Science Program Committee
following a re-evaluation of the science program in June 2002, and
reconfirmed following another re-evaluation of the program in November
2003. The project entered Phase-B2/C/D in February 2006. As of the
summer 2012, Gaia is in Phase-D (Qualification and Production) and will
be launched in the second half of 2013. 8)9)10)

• In June 2013, ESA's
billion-star surveyor, Gaia, has completed final preparations in Europe
and is ready to depart for its launch site in French Guiana. The Gaia
spacecraft arrived in Cayenne, French Guiana, on August 23, 2013 on
board the Antonov 124 aircraft.

• On Oct. 23, 2013, ESA
postponed the launch of the Gaia mission. The decision was taken due to
a technical issue that was identified in another satellite already in
orbit. The issue concerns components used in two transponders on Gaia
that generate ‘timing signals’ for downlinking the science
telemetry. To avoid potential problems, they will be replaced.

The transponders were removed from
Gaia at Kourou and returned to Europe, where the potentially faulty
components were replaced and verified. After the replacements have been
made, the transponders will be refitted to Gaia and a final
verification test made. As a consequence of these precautionary
measures, it will not be possible to launch Gaia within the window that
includes the previously targeted launch date of 20 November. The next
available launch window is 17 December to 5 January 2014. 11)

• Update Oct. 20, 2013: The
upcoming launch manifest of Arianespace has now been established. Gaia
is scheduled for launch on 20 December.

• Update Nov. 22, 2013: The
checks on the Gaia satellite are proceeding well, enabling the launch
to take place on December 19, 2013 (Ref. 11).

Some astrometry basics:

The precise measurement of a
celestial object’s position is known as astrometry, and since
humans first started studying the sky, astronomers have devoted much of
their time to this art. However, Gaia will do so with extraordinary
precision, far beyond the dreams of those ancient astronomers (Ref. 21). 12)

By comparing Gaia’s series of
precise observations, today’s astronomers will soon be able to
make precise measurements of the apparent movement of a star across the
heavens, enabling them to determine its distance and motion through
space. The resulting database will allow astronomers to trace the
history of the Milky Way.

In the course of charting the sky,
Gaia’s highly superior instruments are expected to uncover vast
numbers of previously unknown celestial objects, as well as studying
normal stars. Its expected haul includes asteroids in our Solar System,
icy bodies in the outer Solar System, failed stars, infant stars,
planets around other stars, far-distant stellar explosions, black holes
in the process of feeding and giant black holes at the centers of other
galaxies. Gaia will be a discovery machine.

Stars as individuals and collectives:

To understand fully the physics of a
star, its distance from Earth must be known. This is more difficult
than it sounds because stars are so remote. Even the closest one is 40
trillion km away, and we cannot send spacecraft out to them to measure
as they go. Nor can we bounce radar signals off them, which is the
method used to measure distances within the Solar System. Instead,
astronomers have developed other techniques for measuring and
estimating distances.

The most reliable and only direct
way to measure the distance of a star is by determining its 'parallax'.
By obtaining extremely precise measurements of the positions of stars,
Gaia will yield the parallax for one billion stars; more than 99% of
these have never had their distances measured accurately. Gaia will
also deliver accurate measurements of other important stellar
parameters, including the brightness, temperature, composition and
mass. The observations will cover many different types of stars and
many different stages of stellar evolution.

Figure 2: Distance to a star can
be calculated with simple trigonometry from the measured parallax angle
(1 a.u. is 1 Astronomical Unit, or 149.6 million km), image credit:
ESA/Medialab

The principles of Gaia:

At its heart, Gaia is a space
telescope – or rather, two space telescopes that work as one.
These two telescopes use ten mirrors of various sizes and surface
shapes to collect, focus and direct light to Gaia’s instruments
for detection. The main instrument, an astrometer, precisely determines
the positions of stars in the sky, while the photometer and
spectrometer spread their light out into spectra for analysis.

Gaia’s telescopes point at two
different portions of the sky, separated by a constant 106.5º.
Each has a large primary mirror with a collecting area of about 0.7 m2.
On Earth we are used to round telescope mirrors, but Gaia’s will
be rectangular to make the most efficient use of the limited space
within the spacecraft. These are not large mirrors by modern
astronomical standards, but Gaia’s great advantage is that it
will be observing from space, where there is no atmospheric disturbance
to blur the images. A smaller telescope in space can yield more
accurate results than a large telescope on Earth.

Gaia is just 3.5 m
across, so three curved mirrors and three flat ones are used to focus
and repeatedly fold the light beam over a total path of 35 m before the
light hits the sensitive, custom-made detectors. Together, Gaia’s
telescopes and detectors will be powerful enough to detect stars up to
400,000 times fainter than those visible to the naked eye.

Gaia uses the global astrometry
concept demonstrated by Hipparcos. The principle is to link stars with
large angular distances in a network where each star is connected to a
large number of other stars in every direction. The condition of
closure of the network ensures the reduction of the position errors of
all stars. This is achieved by the simultaneous observation of two
fields of views separated by a very stable basic angle. The spacecraft
is slowly rotating at a constant angular rate of 1º/min around a
spin axis perpendicular to both fields of view, which describe a great
circle on the sky in 6 hours. The spacecraft rotation axis makes an
angle of 45º with the Sun direction (Figure 3).
A slow precession around the Sun-to-Earth direction, with a 63.12 days
period, enables to repeat the observation of sky objects with 86
transits on average over the 5 years of mission.

The resulting
performance will enable a breakthrough in the astrometry field, as well
regarding star position and velocity performance as for the number of
objects observed.

Figure 4: Gaia will improve the
accuracy of astrometry measurements by several orders of magnitude
compared with previous systems and observations (image credit: ESA)

Spacecraft:

Gaia is an exceptionally complex
space observatory. ESA awarded Airbus Defence and Space (former Astrium
SAS,Toulouse, France) the prime contract in May 2006 to develop and
build the spacecraft. Together with the German and British branches of
Astrium, more than 50 industrial subcontractor companies from across
Europe are involved in building this discovery machine. The Gaia DPAC
(Data Processing and Analysis Consortium) will process the raw data to
be published in the largest stellar catalog ever made. 13)14)15)16)17)18)19)20)

The Gaia spacecraft is composed of
two sections: the Payload Module and the Service Module. The Payload
Module is housed inside a protective dome and contains the two
telescopes and the three science instruments. They are all mounted on a
torus made of a ceramic material (silicon carbide). The extraordinary
measurement accuracy required from Gaia calls for an extremely stable
Payload Module that will barely move or deform once in space; this is
achieved thanks to the extensive use of this material. 21)

Underneath the Payload Module, the
Service Module contains electronic units to run the instruments, as
well as the propulsion system, communications units and other essential
components. These components are mounted on CFRP (CarbonFiber
Reinforced Plastic) panels in a conical framework.

Finally, beneath the Service Module,
a large sunshield keeps the spacecraft in shadow, maintaining the
Payload Module at an almost constant temperature of around -110ºC,
to allow the instruments to take their precise and sensitive readings.
The sunshield measures about 10 m across, too large for the launch
vehicle fairing, so it comprises a dozen folding panels that will be
deployed after launch. Some of the solar array panels that are needed
to generate power are fixed on the sunshield, with the rest on the
bottom of the spacecraft.

The Gaia spacecraft configuration is
driven by the required very high thermo-mechanical stability of the
entire spacecraft. A low disturbance cold gas micro-propulsion is used
for fine attitude control. The astrometric instrument is used for
precise rate sensing in fine pointing operating mode.

The SVM is generally referred to as
'the platform'. The SVM in turn is comprised of MSM (Mechanical Service
Module) and an ESM (Electrical Service Module). 22)23)24)

MSM
(Mechanical Service Module): The spacecraft main structure is of
hexagonal conical shape. It is a sandwich panel structure with CFRP
(Carbon Fiber Reinforced Plastic) face sheets, and a central cone
supporting the propellant tanks. The MSM houses instruments needed for
the basic control and operation of the satellite; this includes all
mechanical, structural and thermal elements that support the instrument
payload and spacecraft electronics. It also includes the chemical &
micro propulsion systems, the deployable sunshield with solar arrays,
the payload thermal tent and harness. The module consists of a central
tube that is about 1.17 m long and hosts six radial panels to create a
hexagonal spacecraft shape.

The service module also houses the
communication subsystem, central computer and data handling subsystem,
the high rate data telemetry, attitude control and star trackers. For
telemetry and telecommand, low gain antenna uplink and downlink with a
few kbit/s capacity are employed. The high gain antenna used for the
science telemetry downlink will be used during each ground station
visibility period of an average of about 8 hours per day.

Figure 6: Photo of the SVM integration (image credit: EADS Astrium)

ESM (Electrical Service
Module): The ESM design is driven by the science performance (attitude
control laws with the hybridization of star tracker and payload
measurements, high rate data telemetry, and regulated power bus for
thermal stability). It houses the AOCS units, the communication
subsystem, central computer and data handling subsystem, and the power
subsystem.

Figure 7: Diagram of the ESM (image credit: EADS Astrium)

AOCS (Attitude and Orbit Control Subsystem). The AOCS subsystem is characterized by:

- High precision 3-axis control

- The ASTRO (Astrometric) instrument is used for precise rate sensing during the fine pointing operational mode

- A high precision gyroscope is used
for quick and efficient transitions during the fine pointing
operational mode. Three FOGs (Fiber Optics Gyroscopes) use the
interference of light to detect mechanical rotation. Each unit contains
four closed-loop gyroscope channels to provide built-in redundancy.

- Rugged flight-proven initial acquisition and safe modes

- Three sun acquisition sensors plus one gyroscope provide spin-axis stabilization during the L2 transfer phase of the mission

- One large field of view star
sensor plus use of the main instrument SM (Sky Mapper) for the 3-axis
controlled operational phase.

Gaia AOCS architecture is based on a
fully redundant set of equipment. Moving parts on board are strictly
minimized (e.g. no reaction wheels, no mechanically steerable antenna).
The data is downlinked through a novel electromagnetically steerable
phased array antenna and attitude control is provided by a micro
propulsion system that has its first flight use with Gaia. An atomic
clock is used for precise time-stamping.

Two Autonomous Star Trackers are
used in cold redundancy three FSS (Fine Sun Sensors) are used in hot
redundancy through triple majority voting. Three Gyro packages provide
coarse rate measurements where each gyro package comprises two fully
independent co-aligned channels (i.e. a fiber optic gyroscope sensor
plus associated electronics per channel), with the channels being used
in cold redundancy. As mentioned above, the payload module provides
very precise rate measurements when the spacecraft is operated in fine
pointing science modes. 25)

For actuators, a bi-propellant
Chemical Propulsion System (2 x 8 10 N thrusters used in cold
redundancy) is used for orbit maintenance and attitude control in
coarse AOCS modes (circa 350 kg MON + MMH).

The
Micro-propulsion System (2 x 6 proportional micro-thrusters) can
provide a range of 0 - 1000 µN at a resolution of 0.1 µN.
The individual thrusters are driven by the micro-propulsion electronic,
which is internally redundant and used in cold redundancy (circa 57 kg
of GN2). The nominal science AOCS mode uses the cold gas micro-propulsion system.

The PDHU (Payload Data Handling
Unit) is, among other things, the 'hard-disk' of Gaia, responsible for
temporary storage of science data received from the telescope before
transmission back to Earth. It will receive thousands of compressed
images per second from the observing system; this data will be sorted
and stored. The individual star data objects will be prioritized based
on the magnitude of the star. A complex file management system allows
deletion of low-priority data in the event of data rates or volumes
that exceed the capacity of the storage or transmission systems.

The solid-state storage subsystem of
the PDHU has a capacity of 960 GB which, while not impressive by
terrestrial standards, is extremely large for a space system. It uses a
total of 240 SDRAM modules, each with a capacity of 4 GB, which
populate six memory boards. The PDHU controller board is responsible
for communication with the other spacecraft subsystems, file system
management and the management of telemetry and telecommands. 26)27)

Figure 8: The PDHU (Payload Data Handling Unit), image credit: ESA

The PDHU communicates with the
gigapixel focal plane over seven redundant 40 Mbit/s SpaceWire channels
to acquire the scientific data coming from the seven VPUs (Video
Processing Units) of the camera. The unit's controller sorts the
incoming data according to star magnitude and manages deletion of low
priority data should this become necessary. It sends data for
transmission to Earth under the control of the CDMU (Command and Data
Management Unit). The PDHU communicates with the CDMU via a
MIL-STD-1553 data bus and delivers the science data over two 10 Mbit/s
PacketWire channels.- The PDHU consumes only 26 W, has a mass of 14 kg,
and occupies a volume of 2.3 liter.

EPS (Electrical Power Subsystem): The spacecraft is equipped with a 12.8 m2 high-efficiency triple-junction GaAs (Gallium-Arsenide) cell solar array, of which 7.3 m2 is in the form of a fixed solar array and 5.5 m2 is covered by 6 panels mechanically linked to deployable sunshield assembly.

For the launch, the deployable
sunshield is folded against the payload module. After separation from
the launch vehicle, it is deployed around the fixed solar array, in the
same plane. During LEOP (Launch and Early Operations Phase), power is
supplied by a 60 Ah mass-efficient Lithium-ion battery.

Optimum power supply during all
phases of the mission is ensured by a PCDU (Power Control and
Distribution Unit) with maximum power point tracking. The PCDU performs
power management by generating a 28 V primary power bus that supplies
power to all spacecraft subsystems. It also controls the battery state
of charge and generates pyrotechnic commands as well as heater
actuation as commanded by the C&DMS (Command & Data Management
Subsystem).

Figure 9: Photo of the battery (image credit: ABSL)

Propulsion: After injection
into the L2 transfer orbit by the Soyuz-Fregat launcher, a chemical
bi-propellant propulsion system (8 x 10 N) is used for the transfer
phase. It will cover attitude acquisition, spin control, mid-course
corrections, L2 orbit injection, and safe mode.

After arriving at L2, one redundant
set of micro-propulsion thrusters will control the spin and precession
motion of the spacecraft. Regular orbit maintenance will be performed
by using the chemical propulsion thrusters. - The spacecraft uses a
cold gas micropropulsion system for fine attitude control.

CPS (Chemical Propulsion
Subsystem): CPS is a bi-propellant system using two tanks of
Herschel/Planck heritage filled with with a total of ~400 kg of
propellant featuring a blowdown ratio of 4:3. Use of
monomethylhydrazine as fuel and nitrogen tetroxide as oxidizer. The 10
N thrusters are manufactured by Astrium consisting of a platinum alloy
combustion chamber and nozzle that tolerates the operational
temperature of 1,500°C. The thruster can be operated in a thrust
range of 6 to 12.5 N with a nominal thrust of 10 N which generates a
specific impulse of 291 seconds.

MPS (Micro
Propulsion Subsystem): The MPS is being used for fine attitude pointing
and spin rate management. A total of 12 cold gas thrusters are
installed on the spacecraft being grouped in three clusters each
featuring four cold gas thrusters. The thruster system uses
high-pressure nitrogen propellant to provide very small impulses with a
thrust range of 1 - 500 µN. The system uses two nitrogen tanks,
each containing 28.5 kg of N2, stored at a pressure of 310 bar (Ref. 19). The CG-MPS (Cold Gas-Micro Propulsion Subsystem) was developed by TAS-I and Selex ES S.p.A., Italy. 28)

RF communications: All
communication with the Gaia spacecraft is done using the X-band. For
TT&C (Tracking Telemetry and Command), a low gain antenna uplink
and downlink with a few kbit/s capacity and an omnidirectional coverage
are employed. The science telemetry X-band downlink is based on a set
of electronically-scanned phased array antennae accommodated on the
service module bottom panel. This high gain antenna is used during each
ground station visibility period of about 8 hours per day.

The X-band payload downlink rate is
10 Mbit/s from L2. To achieve this, Gaia uses a specially designed
on-board phased array antenna to beam the payload data to Earth (a
conventional steerable antenna would disturbed the very precise
measurements).

Gaia is equipped with a total of
three communication antennas – two LGAs (Low Gain Antennas) and a
single X-band Medium Gain Phased Array Antenna. One LGA is located
pointing in the +X direction while the other points to –X being
located on the Thermal Tent and the base of the spacecraft,
respectively. The two LGAs build an omni-direction communications
system for housekeeping telemetry downlink and command uplink with data
rates of a few kbit/s.

The MGA (Medium Gain Antenna) is
located on the base of the Payload Module, protruding the
DSA(Deployable Sunshield Assembly) . This directional antenna can
achieve data rates of up to 10 Mbit/s for science data and telemetry
downlink and telecommand reception.

Demodulation of the uplink signal is
completed by the transponder units before the data flow is passed on to
the CDMU (Command & Data Management Unit). The downlink data is
encoded by the CDMU and modulated in X-band within the transponders
before being amplified by the SSPA (Solid State Power Amplifier). The
signal is combined in the phased array of the active antenna in order
to orient the beam towards the Earth.

CCSDS Image Data Compression ASIC:
In order to transmit all the data generated on board, a particularly
challenging compression factor averaging 2.8 was necessary.
Unfortunately the standard suite of algorithms was not able to reach
this target, because of the peculiarities of Gaia imagery, which
include ‘outliers’, such as bright stars and planets, and
which are marred by the momentary ‘hot pixels’ due to
cosmic rays in deep space. Instead, with the support of ESA compression
experts, industry developed an ad hoc solution, enabling all Gaia
mission data to reach their home planet. 29)

CWICOM (CCSDS Wavelet Image
COMpression ASIC) is a very high-performance image compression ASIC
that implements the CCSDS 122.0 wavelet-based image compression
standard, to output compressed data according to the CCSDS output
source packet protocol standard. This integrated circuit was developed
by Airbus DS through an ESA contract.

CWICOM offers dynamic, large
compressed-rate range and high-speed image compression potentially
relevant for compression of any 2D image with bi-dimensional data
correlation (such as a hyperspectral data cube). Its highly optimized
internal architecture allows lossless and lossy image compression at
very high data rates (up to 60 Mpixels/s) without any external memory
by taking advantage of its on-chip memory – almost 5 Mbit of
embedded internal memory).

CWICOM is implemented using the
largest matrix of the Atmel ATC18RHA ASIC family, and is provided
within a standard surface mount package (CQFP 256). CWICOM offers a
low-power, cost-effective and highly integrated solution for any image
compression application, performing CCSDS image compression treatments
without requiring any external memory. The simplicity of such a
standalone implementation is achieved thanks to a very efficient
internal embedded memory organization that removes any need for extra
memory chip procurement and the potential obsolescence threatened by
being bound to a specific external memory interface.

Change in data compression implementation:

In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary.Unfortunately
the standard suite of algorithms [which includes CCSDS standards,
including the 122.0 implemented by CWICOM] was not able to reach this
target, because of the peculiarities of Gaia imagery, which include
‘outliers’, such as bright stars and planets, and which are
marred by the momentary ‘hot pixels’ due to cosmic rays in
deep space. Instead, with the support of ESA compression experts,
industry developed an ad hoc solution,enabling all Gaia mission data to
reach their home planet.

Instead of CWICOM, Gaia applies a
tailored data compression algorithm (using a heavily tailored
pre-processing stage followed by a variant of the Rice coder), using a
software implementation running on VPUs (Video Processing Units). The
GOCA (GAIA Optimum Compression Algorithm) project was entrusted by ESA
to GTD System & Software Engineering (project prime) and IEEC
(Institut d'Estudis Espacials de Catalunya), scientific partner), aimed
in providing a deep understanding of the GAIA compression problem and
offering a complete data compression system, both at an algorithm and
implementation level. The main objectives of GOCA not only encompassed
the review and evaluation of the already proposed compression scheme
but the design of new algorithms for the mission. 30)31)32)

The CCDs in the focal plane are
commanded by video-processing units (VPUs). Gaia has seven identical
VPUs, each one dealing with a dedicated row of CCDs. Each CCD row,
contains in order, two SM CCDs (one for each telescope), 9 AF CCDs, 1
BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven
CCD rows). The VPUs run seven identical instances of the
video-processing algorithms (VPAs), not necessarily with exactly the
same parameter settings though. This (mix of some hardware and mostly)
software is responsible for object detection (after local background
subtraction), object windowing (see below), window conflict resolution,
data binning, data prioritization, science-packet generation, data
compression, etc.) 33)

TCS (Thermal Control
Subsystem): A deployable sunshield with optimal thermoelastic behavior,
made of multi-layer insulation sheets, is attached to the service
module and folded against the payload module for the launch. After
separation of the Gaia spacecraft from the launch vehicle, the Sun
shield is deployed around the fixed solar array, in the same plane. - A
thermal tent covers the payload, offering extra protection against
micrometeoroids and radiation.

The very high
stability thermal control is mostly passive and is achieved through
optical surface reflector material, multilayer insulation sheets on the
outer faces of the service module, and a black painted cavity,
supplemented by heaters where required. Thermal stability is guaranteed
by a constant solar aspect angle and the avoidance (as far as possible)
of any equipment switch-ON/OFF cycles during nominal operation.

DSA (Deployable Sunshield
Assembly): The bottom floor of the SVM is a dodecagonal-shaped panel to
comply with the 12 frames of the DSA The main structure consists of
carbon-fiber reinforced plastic face sheets.

DSA is folded up during launch and
is deployed early in the flight. It is required to shade the payload
unit and protect it from direct sunlight that could compromise
instrument accuracy. Keeping the instrument at a constant temperature
prevents expansion and contraction during temperature variations which
would alter the instrument geometry ever so slightly with a large
effect on data quality. The DSA is 10 m in diameter.

The DSA is an umbrella-type
structure that consists of MLI (Multilayer Insulation) as the primary
shield material and six rigid deployment booms as well as six secondary
stiffeners. These booms have a single articulation on the base of the
Service Module for easy deployment in the radial direction by a spring
system. Spacing cables link the booms to the others to ensure a
synchronized deployment sequence. The booms and strings are located on
the cold side of the cover to limit thermoelastic flexing.

Attached to the DSA are six
rectangular solar panels (with triple-junction solar GaAs cells) that
are constantly facing the sun once the shield is deployed. They provide
1910 W of EOL (End of Life) power.

Figure 12: Photo of Gaia's DSA deployment (image credit: Astrium SAS)

Legend to Figure 12:
The DSA during deployment testing at Astrium Toulouse. Since the DSA
will operate in microgravity, it is not designed to support its own
weight in the one-g environment at Earth's surface. During deployment
testing, the DSA panels are attached to a system of support cables and
counterweights that bears their weight, preventing damage and providing
a realistic test environment. The flight model thermal tent is visible
inside the deploying sunshield and the mechanically representative
dummy payload can be seen through the aperture in the tent.

- All X-band system
- Low Gain Antenna for spacecraft commanding, used also for spacecraft telemetry during critical phases
- Electrically steerable high gain antenna with built in solid state
amplifiers for science downlink up to 8.7 Mbit/s from L2 orbit without
mechanical disturbances

Launch: The GAIA spacecraft
was launched on December 19, 2013 (09:12:19 UTC) from Kourou by
Arianespace, Europe’s Spaceport in French Guiana. The launch
vehicle was a Soyuz-STB with a Fregat-MT upper stage The launch is
designated as Soyuz flight VS06. 35)36)37)

- About ten
minutes later, after separation of the first three stages, the Fregat
upper stage ignited, delivering Gaia into a temporary parking orbit at
an altitude of 175 km.

- A second firing of the Fregat 11
minutes later took Gaia into its transfer orbit, followed by separation
from the upper stage 42 minutes after liftoff. Ground telemetry and
attitude control were established by controllers at ESOC (European
Space Operation Centre) in Darmstadt, Germany, and the spacecraft began
activating its systems.

- The sunshield, which keeps Gaia at
its working temperature and carries solar cells to power the satellite,
was deployed in a 10 minute automatic sequence, completed around 88
minutes after launch. Gaia is now en route towards L2 (Ref. 35).

Orbit: Large Lissajous orbits
around L2 (Lagrangian Point 2), about 1.5 million km from Earth. L2
offers a stable thermal environment because the sunshield will protect
Gaia from the Sun, Earth and Moon simultaneously, allowing the
satellite to keep cool and enjoy a clear view of the Universe from the
other side. In addition, L2 provides a moderate radiation environment,
which benefits the longevity of the instrument detectors.

• The critical LEOP (Launch
and Early Orbit Phase) will last approximately four days. In this
phase, Gaia will perform the first activations – transmitter
switch ON, priming of the chemical thrusters, first attitude control
and finding of the sun position – followed by the sun shield
deployment. Engineers on ground will perform orbit determination, then
prepare and execute the critical 'Day 2' maneuver to inject Gaia into
its final transfer trajectory toward the L2 Lagrange point (Ref. 167).

• LEOP will be followed by the
transfer cruise phase, lasting up to 30 days, an L2 orbit injection
maneuver, then the in-orbit commissioning phase, during which all
operations to prepare for the routine operational phase are performed.
In particular, the scientific FPA (Focal Plane Assembly) and related
avionics will be thoroughly tested and calibrated. The commissioning
phase is expected to last four months.

• The insertion into the final 300,000 x 200,000 km Lissajous orbit around L2 was performed one month after launch.

• August 28, 2019: Rather than
leaving home young, as expected, stellar ‘siblings’ prefer
to stick together in long-lasting, string-like groups, finds a new
study of data from ESA’s Gaia spacecraft. 38)

- Exploring the distribution and
past history of the starry residents of our galaxy is especially
challenging as it requires astronomers to determine the ages of stars.
This is not at all trivial, as ‘average’ stars of a similar
mass but different ages look very much alike.

- To figure out when a star formed,
astronomers must instead look at populations of stars thought to have
formed at the same time – but knowing which stars are siblings
poses a further challenge, since stars do not necessarily hang out long
in the stellar cradles where they formed.

Figure 20: Gaia tracing starry
strings in the Milky Way . This simulated video shows ESA’s Gaia
spacecraft as it traces the structure and star formation activity of a
large patch of space surrounding the Solar System. Gaia launched in
2013, and is on a mission to chart a three-dimensional map of our
galaxy, pinpointing the locations, motions, and dynamics of roughly one
percent of the stars within the Milky Way – along with additional
information about many of these stars. - The video begins with a view
of Gaia set against the bright plane of the Milky Way, which cuts
horizontally across the frame. Different colored patches – each
representing a different stellar ‘family’ observed by Gaia
– then come into view, with yellows, greens, blues, purples and
reds gradually filling up the region and creating a rainbow patchwork
effect. Each family is identified with a different color and comprises
a population of stars that formed at the same time. - Gaia then
disappears from view, and the perspective zooms out to show the wider
three-dimensional structure of the colorful star populations, along
with their future paths through the galaxy based on Gaia’s
measurements of proper motions (the motions have been speeded up for
illustration purposes, with each second corresponding to 158730 years),
video credit: ESA/Gaia/DPAC; Data: M. Kounkel & K. Covey (2019);
Animation: S. Jordan / T. Sagristá / Gaia Sky (http://www.zah.uni-heidelberg.de/gaia/outreach/gaiasky) – CC BY-SA 3.0 IGO

Figure 21: This diagram shows a
face-on view of stellar ‘families’ – clusters (dots)
and co-moving groups (thick lines) of stars – within about 3000
light-years from the Sun, which is located at the center of the image.
The diagram is based on data from the second data release of
ESA’s Gaia mission. Each family is identified with a different
color and comprises a population of stars that formed at the same time.
Purple hues represent the oldest stellar populations, which formed
around 1 billion years ago; blue and green hues represent intermediate
ages, with stars that formed hundreds of millions of years ago; orange
and red hues show the youngest stellar populations, which formed less
than a hundred million years ago. Thin lines show the predicted
velocities of each group of stars over the next 5 million years, based
on Gaia’s measurements. The lack of structures at the center is
an artefact of the method used to trace individual populations, not due
to a physical bubble (image credit: M. Kounkel & K. Covey (2019))

- “To
identify which stars formed together, we look for stars moving
similarly, as all of the stars that formed within the same cloud or
cluster would move in a similar way,” says Marina Kounkel of
Western Washington University, USA, and lead author of the new study.
The study uses data from Gaia’s second release (DR2), provided in April 2018. 39)

- “We knew of a few such
‘co-moving’ star groups near the Solar System, but Gaia
enabled us to explore the Milky Way in great detail out to far greater
distances, revealing many more of these groups.”

- Marina used data from Gaia’s
second release to trace the structure and star formation activity of a
large patch of space surrounding the Solar System, and to explore how
this changed over time. This data release, provided in April 2018,
lists the motions and positions of over one billion stars with
unprecedented precision.

- The analysis of the Gaia data,
relying on a machine learning algorithm, uncovered nearly 2000
previously unidentified clusters and co-moving groups of stars up to
about 3000 light years from us – roughly 750 times the distance
to Proxima Centauri, the nearest star to the Sun. The study also
determined the ages for hundreds of thousands of stars, making it
possible to track stellar ‘families’ and uncover their
surprising arrangements.

Figure 22: This image shows a
view of stellar ‘families’ – clusters and co-moving
groups of stars in the Milky Way – identified using data from the
second data release of ESA’s Gaia mission. Families younger than
30 million years are highlighted in orange, on top of an all-sky view
based on Gaia observations [image credit: ESA/Gaia/DPAC; Data: M.
Kounkel & K. Covey (2019)]

- “Around
half of these stars are found in long, string-like configurations that
mirror features present within their giant birth clouds,” adds
Marina.

- “We generally thought young
stars would leave their birth sites just a few million years after they
form, completely losing ties with their original family – but it
seems that stars can stay close to their siblings for as long as a few
billion years.”

- The strings also appear to be
oriented in particular ways with respect to our galaxy’s spiral
arms – something that depends upon the ages of the stars within a
string. This is especially evident for the youngest strings, comprising
stars younger than 100 million years, which tend to be oriented at
right angles to the spiral arm nearest to our Solar System.

- The astronomers suspect that the
older strings of stars must have been perpendicular to the spiral arms
that existed when these stars formed, which have now been reshuffled
over the past billion years.

Figure 23: This diagram shows an
edge-on view of stellar ‘families’ – clusters (dots)
and co-moving groups (thick lines) of stars – within about 3000
light-years from the Sun, which is located at the center of the image.
The diagram is based on data from the second data release of
ESA’s Gaia mission [image credit: M. Kounkel & K. Covey
(2019)]

- “The proximity and
orientation of the youngest strings to the Milky Way’s
present-day spiral arms shows that older strings are an important
‘fossil record’ of our galaxy’s spiral
structure,” says co-author Kevin Covey, also of Western
Washington University, USA.

- “The nature of spiral arms
is still debated, with the verdict on them being stable or dynamic
structures not settled yet. Studying these older strings will help us
understand if the arms are mostly static, or if they move or dissipate
and re-form over the course of a few hundred million years –
roughly the time it takes for the Sun to orbit around the galactic
center a couple of times.”

• July 25, 2019: On 31 March
2017, Jupiter’s moon Europa passed in front of a background star
– a rare event that was captured for the first time by
ground-based telescopes thanks to data provided by ESA’s Gaia
spacecraft. 40)

- Previously, observatories had only
managed to watch two of Jupiter’s other moons – Io and
Ganymede – during such an event.

- Gaia has been operating in space
since late 2013. The mission aims to produce a three-dimensional map of
our Galaxy, and characterize the myriad stars that call the Milky Way
home. It has been immensely successful so far, revealing the locations and motions of over one billion stars.

- Knowing the precise locations of
the stars we see in the sky allows scientists to predict when various
bodies in the Solar System will appear to pass in front of a background
star from a given vantage point: an event known as a stellar
occultation.

- Occultations are hugely valuable;
they enable measurements of the characteristics of the foreground body
(size, shape, position, and more), and can reveal structures like
rings, jets, and atmospheres. Such measurements can be made from the
ground – something that Bruno Morgado of the Brazilian National
Observatory and LIneA, Brazil, and colleagues took advantage of to
explore Jupiter’s moon Europa.

Figure 24:
Jupiter's largest moons. This 'family portrait' shows a composite of
images of Jupiter, including it's Great Red Spot, and its four largest
moons. From top to bottom, the moons are Io, Europa, Ganymede and
Callisto. Europa is almost the same size as Earth's moon, while
Ganymede, the largest moon in the Solar System, is larger than planet
Mercury. - While Io is a volcanically active world, Europa, Ganymede
and Callisto are icy, and may have oceans of liquid water under their
crusts. Europa in particular may even harbor a habitable environment.
Jupiter and its large icy moons will provide a key focus for ESA's
Juice mission. The spacecraft will tour the Jovian system for about
three-and-a-half years, including flybys of the moons. It will also
enter orbit around Ganymede, the first time any moon beyond our own has
been orbited by a spacecraft. The images of Jupiter, Io, Europa and
Ganymede were taken by NASA's Galileo probe in 1996, while the Callisto
image is from the 1979 flyby of Voyager (image via NASA Photojournal)

- “We used data from
Gaia’s first data release to forecast that, from our viewpoint in
South America, Europa would pass in front of a bright background star
in March 2017 – and to predict the best location from which to
observe this occultation,” said Bruno, lead researcher of a new
paper reporting the findings from the 2017 occultation. Gaia’s
first data release was provided in September 2016. 41)

- “This gave us a wonderful
opportunity to explore Europa, as the technique offers an accuracy
comparable to that of images obtained by space probes.”

- The Gaia data showed that the
event would be visible from a thick band slicing from north-west to
south-east across South America. Three observatories located in Brazil
and Chile were able to capture data – a total of eight sites
attempted, but many experienced poor weather conditions.

- In-keeping with previous
measurements, the observations refined Europa’s radius to 1561.2
km, precisely determined Europa’s position in space and in
relation to its host planet, Jupiter, and characterized the
moon’s shape. Rather than being exactly spherical, Europa is
known to be an ellipsoid. The observations show the moon to measure
1562 km when measured across in one direction (the so-called apparent
‘semi-major’ axis), and 1560.4 km when measured across the
other (the apparent ‘semi-minor’ axis).

- “It’s likely that
we’ll be able to observe far more occultations like this by
Jupiter’s moons in 2019 and 2020,” adds Bruno.
“Jupiter is passing through a patch of sky that has the galactic
center in the background, making it drastically more likely that its
moons will pass in front of bright background stars. This would really
help us to pin down their three-dimensional shapes and positions
– not only for Jupiter's four largest moons, but for smaller,
more irregularly-shaped ones, too.”

- Using Gaia’s second data
release, provided in April 2018, the scientists predict the dates of
further occultations of bright stars by Europa, Io, Ganymede and
Callisto in coming years, and list a total of 10 events through 2019
and 2021. Future events comprise stellar occultations by Europa (22
June 2020), Callisto (20 June 2020, 4 May 2021), Io (9 and 21 September
2019, 2 April 2021), and Ganymede (25 April 2021).

- Three have already taken place in
2019, two of which – stellar occultations by Europa (4 June) and
Callisto (5 June) – were also observed by the researchers, and
for which the data are still under analysis.

- The upcoming occultations will be
observable even with amateur telescopes as small as 20 cm from various
regions around the world. The favorable position of Jupiter, with the
galactic plane in the background, will only occur again in 2031.

Figure 25: Upcoming stellar
occultations by Jupiter’s four largest moons. Astronomers can
learn a great deal about a celestial body by observing it as it moves
in front of a bright background star: an alignment known as a stellar
occultation. Such events are unusual for Jupiter’s moons. In
fact, until recently, only two of the gas giant’s moons –
Io and Ganymede – had been observed during stellar occultations.
Now, a study presents observations of another of Jupiter’s moons,
Europa, as it obscured a bright star on 31 March 2017. This event
allowed the astronomers to better characterize Europa’s size,
position in space and in relation to Jupiter, and three-dimensional
shape; they used precise data from Gaia’s first data release,
provided in September 2016, on stellar positions to determine the best
location from which to observe the event, and subsequently gathered
data from three observatories in Brazil and Chile [image credit:
ESA/Gaia/DPAC; Bruno Morgado (Brazilian National Observatory/LIneA,
Brazil) et al (2019)]

- “Stellar occultation studies
allow us to learn about moons in the Solar System from afar, and are
also relevant for future missions that will visit these worlds,”
says Timo Prusti, ESA Gaia Project Scientist. “As this result
shows, Gaia is a hugely versatile mission: it not only advances our
knowledge of stars, but also of the Solar System more widely.”

- An accurate knowledge of
Europa’s orbit will help to prepare space missions targeting the
Jovian system such as ESA’s JUICE (JUpiter ICy moons Explorer) and NASA’s Europa Clipper, both of which are scheduled for launch in the next decade.

Figure 26: Juice's Europa flyby.
ESA’s Jupiter Icy Moons Explorer, Juice, is set to embark on a
seven-year cruise to Jupiter starting May 2022. The mission will
investigate the emergence of habitable worlds around gas giants and the
Jupiter system as an archetype for the numerous giant planets now known
to orbit other stars. During the tour of the Jovian system, Juice will
make two flybys of Europa, which has strong evidence for an ocean of
liquid water under its icy shell. Juice will look at the moon’s
active zones, its surface composition and geology, search for pockets
of liquid water under the surface and study the plasma environment
around Europa (image credit: ESA)

- “These
kinds of observations are hugely exciting,” says Olivier Witasse,
ESA’s Juice Project Scientist. “Juice will reach Jupiter in
2029; having the best possible knowledge of the positions of the
system’s moons will help us to prepare for the mission navigation
and future data analysis, and plan all of the science we intend to do.

- “This science depends upon
us knowing things such as accurate moon trajectories and understanding
how close a spacecraft will come to a given body, so the better our
knowledge, the better this planning – and the subsequent data
analysis – will be.”

• July 16, 2019: The first
direct measurement of the bar-shaped collection of stars at the center
of our Milky Way galaxy has been made by combining data from
ESA’s Gaia mission with complementary observations from ground-
and space-based telescopes. 42)

Figure 27: Revealing the
galactic bar. This color chart shows the distribution of 150 million
stars in the Milky Way probed using data from the second release of
ESA’s Gaia mission in combination with infrared and optical
surveys, with orange/yellow hues indicating a greater density of stars.
Most of these stars are red giants. While the majority of charted stars
are located closer to the Sun (the larger orange/yellow blob in the
lower part of the image), a large and elongated feature populated by
many stars is also visible in the central region of the galaxy: this is
the first geometric indication of the galactic bar. The distances to
the stars shown in this chart, along with their surface temperature and
extinction – a measure of how much dust there is between us and
the stars – were estimated using the StarHorse computer code
[video credit: Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse
team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)]

- The second release of data from
ESA’s Gaia star-mapping satellite, published in 2018, has been
revolutionizing many fields of astronomy. The unprecedented catalog
contains the brightnesses, positions, distance indicators and motions
across the sky for more than one billion stars in our Milky Way galaxy,
along with information about other celestial bodies.

- As impressive as this dataset
sounds, this is really just the beginning. While the second release is
based on the first 22 months of Gaia’s surveys, the satellite has
been scanning the sky for five years and has many years ahead. New data
releases planned in the coming years will steadily improve measurements
as well as provide extra information that will enable us to chart our
home galaxy and delve into its history like never before.

- Meanwhile, a team of astronomers
have combined the latest Gaia data with infrared and optical
observations performed from ground and space to provide a preview of
what future releases of ESA’s stellar surveyor will reveal.

- “We looked in particular at
two of the stellar parameters contained in the Gaia data: the surface
temperature of stars and the ‘extinction’, which is
basically a measure of how much dust there is between us and the stars,
obscuring their light and making it appear redder,” says
Friedrich Anders from University of Barcelona, Spain, lead author of
the new study. “These two parameters are interconnected, but we
can estimate them independently by adding extra information obtained by
peering through the dust with infrared observations.”

- The team combined the second Gaia
data release with several infrared surveys using a computer code called
StarHorse, developed by co-author Anna Queiroz and collaborators. The
code compares the observations with stellar models to determine the
surface temperature of stars, the extinction and an improved estimate
of the distance to the stars.

- As a result, the astronomers
obtained a much better determination of the distances to about 150
million stars – in some cases, the improvement is up to 20% or
more. This enabled them to trace the distribution of stars across the
Milky Way to much greater distances than possible with the original
Gaia data alone.

- “With the second Gaia data
release, we could probe a radius around the Sun of about 6500 light
years, but with our new catalog, we can extend this ‘Gaia
sphere’ by three or four times, reaching out to the center of the
Milky Way,” explains co-author Cristina Chiappini from Leibniz
Institute for Astrophysics Potsdam, Germany, where the project was
coordinated.

- There, at the center of our
galaxy, the data clearly reveals a large, elongated feature in the
three-dimensional distribution of stars: the galactic bar.

- “We know the Milky Way has a
bar, like other barred spiral galaxies, but so far we only had indirect
indications from the motions of stars and gas, or from star counts in
infrared surveys. This is the first time that we see the galactic bar
in 3D space, based on geometric measurements of stellar
distances,” says Friedrich.

- “Ultimately, we are
interested in galactic archeology: we want to reconstruct how the Milky
Way formed and evolved, and to do so we have to understand the history
of each and every one of its components,” adds Cristina.

- “It is still unclear how the
bar – a large amount of stars and gas rotating rigidly around the
center of the galaxy – formed, but with Gaia and other upcoming
surveys in the next years we are certainly on the right path to figure
it out.”

- The team is looking forward to the
next data release from the Apache Point Observatory Galaxy Evolution
Experiment (APOGEE-2), as well as upcoming facilities such as the
4-meter Multi-Object Survey Telescope (4MOST) at the European Southern
Observatory in Chile and the WEAVE (WHT Enhanced Area Velocity
Explorer) survey at the William Herschel Telescope (WHT) in La Palma,
Canary Islands.

- The third Gaia data release,
currently planned for 2021, will include greatly improved distance
determinations for a much larger number of stars, and is expected to
enable progress in our understanding of the complex region at the
center of the Milky Way.

Figure 28: Revealing the
galactic bar. This color chart shows the distribution of 150 million
stars in the Milky Way probed using data from the second release of
ESA’s Gaia mission in combination with infrared and optical
surveys, with orange/yellow hues indicating a greater density of stars.
Most of these stars are red giants. The distribution is superimposed on
an artistic top view of our galaxy. - While the majority of charted
stars are located closer to the Sun (the larger orange/yellow blob in
the lower part of the image), a large and elongated feature populated
by many stars is also visible in the central region of the galaxy: this
is the first geometric indication of the galactic bar. - The distances
to the stars shown in this chart, along with their surface temperature
and extinction – a measure of how much dust there is between us
and the stars – were estimated using the StarHorse computer code
[image credit: Data: ESA/Gaia/DPAC, A. Khalatyan (AIP) & StarHorse
team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)]

- “With this study, we can
enjoy a taster of the improvements in our knowledge of the Milky Way
that can be expected from Gaia measurements in the third data
release,” explains co-author Anthony Brown of Leiden University,
The Netherlands, and chair of the Gaia Data Processing and Analysis
Consortium Executive.

- “We are revealing features
in the Milky Way that we could not see otherwise: this is the power of
Gaia, which is enhanced even further in combination with complementary
surveys,” concludes Timo Prusti, Gaia project scientist at ESA.

- The study combines data from
Gaia’s second release with the Pan-STARRS1 survey conducted with
the first Pan-STARRS telescope in Hawaii, US; the Two Micron All Sky
Survey (2MASS) conducted with telescopes in the US and Chile; the
AllWISE survey from NASA’s Wide-field Infrared Survey Explorer
(WISE).

• July 15, 2019: On Tuesday 16 July, teams at ESA’s mission control will perform an ‘orbit change maneuver’ on the Gaia space observatory – the biggest operation since the spacecraft was launched in 2013. 44)

- Gaia is on a mission to survey
more than a billion stars, charting the largest three-dimensional map
of our galaxy, the Milky Way. In so doing, the spacecraft is revealing
the composition, formation and evolution of our galaxy, and a whole lot
more.

- For the last five and a half
years, the spacecraft has travelled in an orbit designed to keep it out
of Earth’s shadow, the second Lagrange point.

- At 1.5 million km from Earth
– four times further than the Moon – the ‘L2’
is a fabulous place from which to do science. As the Sun, Earth and
Moon are all in one direction relative to the spacecraft, the rest of
the sky is free to observe.

- Placing Gaia in L2 has also
ensured the star-catcher’s stability, because to this day it has
never passed into Earth’s shadow. This has kept the spacecraft
undisturbed by any change in temperature or varying infrared radiation
that would result from an Earth eclipse.

- Although at the end of its planned
lifetime, Gaia still has fuel in the tank and a lot more science to do,
and so its mission continues. However, its eclipse-dodging path will
not. In August and November of this year, without measures to change
its orbit, the billion-star hunter will become partially shrouded by
Earth’s shadow.

Figure 29: Avoiding Earth's
shadow. These two eclipses would prevent enough of the Sun’s
light reaching Gaia’s solar panels that the observatory would
shut down. As well as affecting its stability and power, such shade
would cause a thermal disturbance, impacting the spacecraft’s
scientific data acquisition for weeks (image credit: ESA)

Eclipse avoidance

- To keep Gaia safe from these shady
possibilities, operators at ESA’s mission control are planning
the ‘Whitehead eclipse avoidance maneuver’.

- On 16 July, Gaia will use a
combination of its onboard thrusters to push it in a diagonal
direction, away from the shadow, in a special technique known as
'thrust vectoring'.

- “We’ve named this
operation after a great colleague of ours, Gary Whitehead, who sadly
passed away last month after serving on the Flight Control Team for
more than 11 years,” says David Milligan, Spacecraft Operations
Manager for the mission.

- “The maneuver will allow us
to change Gaia’s orbit without having to turn the spacecraft
body, keeping sunlight safely away from its extremely sensitive
telescope.”

The world's most stable space observatory

- Gaia is an incredibly stable
spacecraft. In fact, it is many, many times more stable – and
therefore precise – than any other spacecraft in operation today.

- “In space, stability takes
time to establish,” explains David. “Because any
temperature change or unusual movement could take weeks to diminish or
dampen, we always limit the time where special activities are performed
that disturb scientific observations.”

- “As well as the Whitehead
maneuver, we will perform some maintenance and calibration activities
on the spacecraft’s complex subsystems, which would otherwise
have disturbed Gaia’s science.”

- Because of its position and
unparalleled precision, Gaia is one of the most productive spacecraft
out there. Last year alone, more than 800 scientific papers were
published based on its observations.

• June 28, 2019: Each year on
30 June, the worldwide UN-sanctioned Asteroid Day takes place to raise
awareness about asteroids and what can be done to protect Earth from
possible impact. The day falls on the anniversary of the Tunguska event
that took place on 30 June 1908, the most harmful known asteroid
related event in recent history. 45)

Figure 30: Animated view of more
than 14,000 asteroids in our Solar System from the catalog in the
second data release of ESA’s Gaia satellite, published in 2018.
The orbits of the 200 brightest objects are shown in green. In
addition, the orbits of the first four asteroids discovered by Gaia are
shown in pink (video credit: ESA/Gaia/DPAC – CC BY-SA 3.0 IGO;
Music copyright: Encore 5 by Christophe Goze, audionetwork.com)

- While Gaia’s main scientific
goal is to chart a billion stars in our Milky Way galaxy, the satellite
is also sensitive to celestial bodies closer to home, regularly
observing known asteroids and occasionally discovering new ones.

- Three of the newly discovered
asteroids, temporarily designated as 2018 YK4, 2018 YL4 and 2018 YM4,
were first spotted by Gaia in December 2018, and later confirmed by
follow-up observations performed with the Haute-Provence Observatory in
France, which enabled scientists to determine their orbits. Comparing
these informations with existing observations indicated the objects had
not been detected earlier.

- The fourth discovery, an asteroid
with temporary designation 2019 CZ10, was first detected by Gaia in
February, and was recently confirmed by ground-based observations by
the Mount Lemmon Survey and the Pan-STARRS 1 project in the US.

- These four asteroids, while part
of the ‘main belt’ between the orbits of Mars and Jupiter,
move around the Sun on orbits that have a greater tilt (15 degrees or
more) with respect to the orbital plane of planets than most main-belt
asteroids.

- The population of such
high-inclination asteroids is not as well studied as those with less
tilted orbits, since most surveys tend to focus on the plane where the
majority of asteroids reside. But Gaia can readily observe them as it scans the entire sky
from its vantage point in space, so it is possible that the satellite
will find more such objects in the future and contribute new
information to study their properties.

- Alongside the extensive processing and analysis of Gaia’s data in preparation for subsequent data releases, preliminary information about Gaia’s asteroid detections are regularly shared via an online alert system so that astronomers across the world can perform follow-up observations.

- This animation starts showing the
position of planets, asteroids and stars on Asteroid Day, 30 June 2019;
time has been speeded up by 5 million.

• May 9, 2019: A team led by
researchers of the Institute of Cosmos Sciences of the University of
Barcelona (ICCUB, UB-IEEC) and the Besançon Astronomical
Observatory have analyzed data from the Gaia satellite and found that a
heavy star formation burst occurred in the Milky Way about 3,000
million years ago. During this process, more than 50 percent of the
stars that created the galactic disc may have been born. These results
are derived from the combination of the distances, colors and magnitude
of the stars that were measured by Gaia with models that predict their
distribution in our galaxy. The study has been published in the journal
Astronomy & Astrophysics. 46)47)

- Like a flame fades when there is
no gas in the cylinder, the rhythm of the stellar formation in the
Milky Way, fueled by deposited gas, is predicted to decrease slowly and
in a continuous way as the existing gas is extinguished. The results of
the study show that although this process took place over the first
4,000 million years of Milky Way disc formation, a severe star
formation burst, or "stellar baby boom," inverted this trend. A
gas-rich satellite galaxy merged with the Milky Way, and could have
introduced new fuel and reactivated the process of stellar formation.
This mechanism would explain the distribution of distances, ages and
masses that are estimated from the data taken from the European Space
Agency Gaia satellite.

- "The time scale of this star
formation burst, together with the great stellar mass involved in the
process—thousands of millions of solar masses—suggests the
disc of our galaxy did not have a steady and paused evolution. It may
have suffered an external perturbation that began about 5 billion years
ago," said Roger Mor, ICCUB researcher and first signer of the article.

- "We have been able to find this
out by analyzing the precise distances for more than 3 million stars in
the solar environment," says Roger Mor. "Thanks to these data, we could
discover the mechanisms that controlled the evolution more than 8 to 10
billion years ago in the disc of our galaxy, which is not more than the
bright band we see in the sky on a dark night and with no light
pollution." Like in many research fields these days, these findings are
possible thanks to the availability of the combination of a great
amount of unprecedented precision data, and many hours of computing.

- Cosmologic
models predict our galaxy would have been growing due the merging with
other galaxies, a fact that has been stated by other studies using Gaia
data. One of these merges could be the cause of the severe star
formation burst that was detected in this study.

- "Actually, the peak of star
formation is so clear, unlike predictions from before Gaia data
availability, that we thought it necessary to treat its interpretation
together with experts on cosmological evolution of external galaxies,"
notes Francesca Figuerars, lecturer at the Department of Quantum
Physics and Astrophysics of the UB, ICCUB member and signer of the
article.

- Santi Roca-Fàbrega from the
Complutense University of Madrid, an expert in stellar modeling and
co-author, said, "The obtained results match with what the current
cosmological models predict, and what is more, our galaxy seen from
Gaia's eyes is an excellent cosmological laboratory where we can test
and confront models at a bigger scale in the universe."

Figure 31: The region of the
stellar formation Rho Ophiuchi observed by ESA Gaia satellite. The
shining dots are stellar clusters with the massive and youngest stars
of the region. The dark filaments track the gas and dust distribution,
where the new stars are born. This is not a conventional photographic
image but the result of the integration of all the received radiation
by the satellite during the 22 months of continuous measurements
through different filters on the spacecraft (image credit:
ESA/Gaia/DPAC, CC BY-SA 3.0 IGO)

Figure 32:
Distribution of 3 million stars used in this study to detect the star
formation burst from 2-3 billion years ago. Gaia provided the distance
for each of these objects on the galactic disc. Shown is a scheme of
the spiral arms of the Milky Way (image credit: University of Barcelona)

• May 2, 2019: While
ESA’s Gaia mission has been surveying more than one billion stars
from space, astronomers have been regularly monitoring the
satellite’s position in the sky with telescopes across the world,
including the European Southern Observatory in Chile, to further refine
Gaia’s orbit and ultimately improve the accuracy of its stellar
census. 48)49)

- One year ago, the Gaia mission released its much-awaited second set of data,
which included high-precision measurements – positions, distance
indicators and proper motions – of more than one billion stars in
our Milky Way galaxy.

- The catalog, based on less than
two years of observations and almost four years of data processing and
analysis by a collaboration of about 450 scientists and software
engineers, has enabled transformational studies in many fields of
astronomy, generating more than 1000 scientific publications in the
past twelve months.

Figure 33: Pinpointing Gaia from
Earth. A sequence of images taken as part of the Ground Based Optical
Tracking campaign of ESA’s Gaia satellite with the European
Southern Observatory’s (ESO) 2.6 m VLT Survey Telescope (VST) in
Chile. This image combines ten observations performed on 14 April 2019:
Gaia is visible as a line of ten faint dots just below the image
center. The stars in the image appear as slightly elongated, since the
telescope is following Gaia rather than the stars. The observations
have been stacked using the stars as reference to show the movement of
Gaia across the sky. The images were obtained using the OMEGACAM
instrument with the SDSS-r filter on the VST, with an exposure time of
60 seconds for each individual observation; the whole sequence covers
about 17 minutes. Astrometric determination of Gaia's position is
conducted on each frame, using as reference the coordinates of the
background stars as provided in the second Gaia data release. The
results are then averaged and the resulting value for the position of
Gaia has a precision of 20 mas (milliarcseconds) or better (one
arcsecond is equivalent to the size of a Euro coin seen from a distance
of about 4 km), image credit: ESO; CC BY 4.0

- Meanwhile in space, Gaia keeps
scanning the sky and gathering data that is being crunched for future
releases to achieve even higher precision on the position and motion of
stars and enable ever deeper and more detailed studies into our place
in the cosmos. But to reach the accuracy expected for Gaia’s
final catalogue, it is crucial to pinpoint the position and motion of the satellite from Earth.

- To this aim, the flight dynamics
experts at ESA’s operations center make use of a combination of
techniques, from traditional radio tracking and ranging to simultaneous
observing using two radio antennas – the so-called delta-DOR method.

Figure 35:
Gaia scanning the sky. This animation shows the Gaia spacecraft
spinning in space scanning the sky. Gaia’s mission relies on the
systematic and repeating observation of star positions in two fields of
view. As the detectors repeatedly measure the position of each
celestial object, they will detect any changes in the object’s
motion through space. To achieve its mission the spacecraft is spinning
slowly, sweeping its two telescopes across the entire celestial sphere
to make four complete rotations per day (video credit: ESA – C.
Carreau)

Legend to Figure 35:
Gaia’s telescopes point at two different portions of the sky,
separated by a constant 106.5°. Therefore, objects arrive in the
second field of view 106.5 minutes after they are observed in the
first. Meanwhile its spin axis precesses around the Sun with a period
of about 63 days, allowing different parts of the sky to be scanned.
This scanning strategy builds up an interlocking grid of positions,
providing absolute – rather than relative – values of the
stellar positions and motions. The spacecraft spin axis makes an angle
of 45° with the Sun direction, ensuring that the payload is shaded
from the Sun, but that the solar arrays can still produce electricity
efficiently.

- In a unique and
novel approach for ESA, the ground-based tracking of Gaia also includes
optical observations provided by a network of medium-size telescopes
across the planet.

- The European Southern
Observatory’s (ESO) 2.6 m VLT Survey Telescope (VST) in Chile
records Gaia’s position in the sky for about 180 nights every
year.

- “The VST is the perfect tool
for picking out the motion of Gaia,” adds Ferdinando Patat, head
of the ESO’s Observing Programs Office. “Using one of
ESO’s first-rate ground-based facilities to bolster cutting-edge
space observations is a fine example of scientific cooperation.”

- In addition, the 2 m Liverpool
telescope located on La Palma, Canary Islands, Spain, and the Las
Cumbres Optical Global Telescope Network, which operates 2 m telescopes
in Australia and the US, have also been observing Gaia over the past
five years as part of the Ground Based Optical Tracking (GBOT)
campaign.

- “Gaia observations require a
special observing procedure,” explains Monika Petr-Gotzens, who
has coordinated the execution of ESO’s observations of Gaia since
2013. “The spacecraft is what we call a ‘moving
target’, as it is moving quickly relative to background stars
– tracking Gaia is quite the challenge!”

- In these images Gaia is a mere dot
of light among the many stars that the satellite itself has been
measuring, so painstaking calibration is needed to transform this body
of observations into meaningful data that can be included in the
determination of the satellite’s orbit.

- This required using data from
Gaia’s second release to identify the stars in each of the images
collected over the past five years and calculate the satellite’s
position in the sky with a precision of 20 mas or better (one arcsecond
is equivalent to the size of a Euro coin seen from a distance of about
4 km).

- The ground-based observations also
provide key information to improve the determination of Gaia’s
velocity through space, which must be known to the precision of a few
mm/s. This is necessary to correct for a phenomenon known as aberration
of light – an apparent distortion in the direction of incoming
light due to the relative motion between the source and an observer
– in a way similar to tilting one’s umbrella while walking
through the rain.

- “After careful and lengthy
data processing, we have now achieved the accuracy required for the
ground-based observations of Gaia to be implemented as part of the
orbit determination,” says Martin Altmann, lead of the GBOT
campaign from the Astronomisches Rechen-Institut, Center for Astronomy
of Heidelberg University, Germany, who works in close collaboration
with colleagues from the Paris Observatory in France.

- The GBOT information will be used
to improve our knowledge of Gaia’s orbit not only in observations
to come, but also for all the data that have been gathered from Earth
in the previous years, leading to improvements in the data products
that will be included in future releases.

• April 29, 2019: While scanning the sky to chart a billion stars in our Milky Way galaxy, ESA’s Gaia satellite is also sensitive to celestial bodies closer to home, and regularly observes asteroids 50) in our Solar System. 51)

Figure 36:
Gaia's first asteroid survey. Animated view of the 14,099 asteroids in
our Solar System, as viewed by ESA’s Gaia satellite using
information from the mission’s second data release. The orbits of
the 200 brightest asteroids are also shown, as determined using Gaia
data. - In future data releases, Gaia will also provide asteroid
spectra and enable a complete characterization of the asteroid belt.
The combination of dynamical and physical information that is being
collected by Gaia provides an unprecedented opportunity to improve our
understanding of the origin and the evolution of the Solar System
(video credit: Gaia Data Processing and Analysis Consortium (DPAC);
Orbits: Gaia Coordinating Unit 4; P. Tanga, Observatoire de la
Côte d'Azur, France; F. Spoto, IMCCE, Observatoire de Paris,
France; Animation: Gaia Sky; S. Jordan / T. Sagristà,
Astronomisches Rechen-Institut, Zentrum für Astronomie der
Universität Heidelberg, Germany)

Figure 37: This view shows the orbits of more than 14,000 known asteroids
(with the Sun at the center of the image) based on information from
Gaia’s second data release, which was made public in 2018. The
majority of asteroids depicted in this image, shown in bright red and
orange hues, are main-belt asteroids, located between the orbits of
Mars and Jupiter; Trojan asteroids, found around the orbit of Jupiter,
are shown in dark red.In yellow, towards the image center, are the orbits of several tens of near-Earth asteroids
observed by Gaia: these are asteroids that come to within 1.3
astronomical units (AU) to the Sun at the closest approach along their
orbit. The Earth circles the Sun at a distance of 1 AU (around 150
million km) so near-Earth asteroids have the potential to come into
proximity with our planet (image credit: ESA/Gaia/DPAC)

- Most asteroids that Gaia detects are already known,
but every now and then, the asteroids seen by ESA's Milky Way surveyor
do not match any existing observations. This is the case for the three
orbits shown in grey in this view: these are Gaia’s first
asteroid discoveries.

- The three new asteroids were first
spotted by Gaia in December 2018, and later confirmed by follow-up
observations performed with the Haute-Provence Observatory in France,
which enabled scientists to determine their orbits. Comparing these
informations with existing observations indicated the objects had not
been detected earlier.

- While they are part of the main
belt of asteroids, all three move around the Sun on orbits that have a
greater tilt (15 degrees or more) with respect to the orbital plane of
planets than most main-belt asteroids.

- The population of such
high-inclination asteroids is not as well studied as those with less
tilted orbits, since most surveys tend to focus on the plane where the
majority of asteroids reside. But Gaia can readily observe them as it scans the entire sky
from its vantage point in space, so it is possible that the satellite
will find more such objects in the future and contribute new
information to study their properties.

- Alongside the extensive processing and analysis of Gaia’s data in preparation for subsequent data releases, preliminary information about Gaia’s asteroid detections are regularly shared via an online alert system
so that astronomers across the world can perform follow-up
observations. To observe these asteroids, a 1-m or larger telescope is
needed.

- Once an asteroid detected by Gaia
has been identified also in ground-based observations, the scientists
in charge of the alert system analyze the data to determine the
object’s orbit. In case the ground observations match the orbit
based on Gaia’s data, they provide the information to the Minor Planet Center,
which is the official worldwide organization collecting observational
data for small Solar System bodies like asteroids and comets.

- This process may lead to new
discoveries, like the three asteroids with orbits depicted in this
image, or to improvements in the determination of the orbits of known
asteroids, which are sometimes very poorly known. So far, several tens of asteroids detected by Gaia have been observed from the ground in response to the alert
system, all of them belonging to the main belt, but it is possible that
also near-Earth asteroids will be spotted in the future.

- A number of observatories across
the world are already involved in these activities, including the
Haute-Provence Observatory, Kyiv Comet station, Odessa-Mayaki, Terskol,
C2PU at Observatoire de la Côte d'Azur and Las Cumbres
Observatory Global Telescope Network. The more that join, the more we will learn about asteroids – known and new ones alike.

• March 7, 2019: In a striking
example of multi-mission astronomy, measurements from the NASA/ESA
Hubble Space Telescope and the ESA Gaia mission have been combined to
improve the estimate of the mass of our home galaxy the Milky Way: 1.5
trillion (1.5 x 1012) solar masses. 52)

Figure 38: This artist's
impression shows a computer generated model of the Milky Way and the
accurate positions of the globular clusters used in this study
surrounding it. Scientists used the measured velocities of these 44
globular clusters to determine the total mass of the Milky Way, our
cosmic home. Satellite: Hubble Space Telescope (image credit:
ESA/Hubble, NASA, L. Calçada)

- The mass of the Milky Way is one
of the most fundamental measurements astronomers can make about our
galactic home. However, despite decades of intense effort, even the
best available estimates of the Milky Way's mass disagree wildly. Now,
by combining new data from the European Space Agency (ESA) Gaia mission
with observations made with the NASA/ESA Hubble Space Telescope,
astronomers have found that the Milky Way weighs in at about 1.5
trillion solar masses within a radius of 129,000 light-years from the
galactic center.

- Previous estimates of the mass of the Milky way ranged from 500 billion (500 x 109) to 3 trillion (3 x 1012)
times the mass of the Sun. This huge uncertainty arose primarily from
the different methods used for measuring the distribution of dark
matter – which makes up about 90% of the mass of the galaxy.

- "We just can't detect dark matter
directly," explains Laura Watkins (European Southern Observatory,
Germany), who led the team performing the analysis. "That's what leads
to the present uncertainty in the Milky Way's mass – you can't
measure accurately what you can't see!"

- Given the
elusive nature of the dark matter, the team had to use a clever method
to weigh the Milky Way, which relied on measuring the velocities of
globular clusters – dense star clusters that orbit the spiral
disc of the galaxy at great distances.
Note: Globular clusters formed prior to the construction of the Milky
Way's spiral disk, where our Sun and the Solar System later formed.
Because of their great distances, globular star clusters allow
astronomers to trace the mass of the vast envelope of dark matter
surrounding our galaxy far beyond the spiral disk.

- "The more massive a galaxy, the
faster its clusters move under the pull of its gravity" explains N. Wyn
Evans (University of Cambridge, UK). "Most previous measurements have
found the speed at which a cluster is approaching or receding from
Earth, that is the velocity along our line of sight. However, we were
able to also measure the sideways motion of the clusters, from which
the total velocity, and consequently the galactic mass, can be
calculated."
Note: The total velocity of an object is made up of three motions
– a radial motion plus two defining the sideway motions. However,
in astronomy most often only line-of-sight velocities are available.
With only one component of the velocity available, the estimated masses
depend very strongly on the assumptions for the sideway motions.
Therefore measuring the sideway motions directly significantly reduces
the size of the error bars for the mass.

- The group used Gaia's second data release
as a basis for their study. Gaia was designed to create a precise
three-dimensional map of astronomical objects throughout the Milky Way
and to track their motions. Its second data release includes
measurements of globular clusters as far as 65,000 light-years from
Earth.

- "Global clusters extend out to a
great distance, so they are considered the best tracers astronomers use
to measure the mass of our galaxy" said Tony Sohn of STScI (Space
Telescope Science Institute), Baltimore, MD, USA, who led the Hubble
measurements.

- The team combined these data with
Hubble's unparalleled sensitivity and observational legacy.
Observations from Hubble allowed faint and distant globular clusters,
as far as 130,000 light-years from Earth, to be added to the study. As
Hubble has been observing some of these objects for a decade, it was
possible to accurately track the velocities of these clusters as well.

- "We were lucky to have such a
great combination of data," explained Roeland P. van der Marel of
STScI. "By combining Gaia's measurements of 34 globular clusters with
measurements of 12 more distant clusters from Hubble, we could pin down
the Milky Way's mass in a way that would be impossible without these
two space telescopes."

- Until now, not knowing the precise
mass of the Milky Way has presented a problem for attempts to answer a
lot of cosmological questions. The dark matter content of a galaxy and
its distribution are intrinsically linked to the formation and growth
of structures in the Universe. Accurately determining the mass for the
Milky Way gives us a clearer understanding of where our galaxy sits in
a cosmological context. 53)

• February 26, 2019: ESA's
Gaia satellite is on a mission: to map and characterize more than one
billion of the stars in the Milky Way. Many of these stars reside in
complex, eye-catching clusters scattered throughout our Galaxy and, by
studying these stellar groupings, Gaia is revealing much about the
formation and evolution of stars in our cosmic home and surroundings. 54)

- The Milky Way is full of stars.
Our Galaxy contains over a hundred billion of them, from dwarf to
giant, populating its crowded center and its spiralling disc.

- Many of these stars are thought to
have formed in the same way: from huge clouds of cool, condensing
molecular gas, which collapse under the influence of gravity and
fragment to form groups of hundreds to thousands of stars, known as
star clusters. Some of these clusters last thousands of millions of
years, while others disperse rapidly, releasing their stellar residents
into the Milky Way's disc.

- It is likely that also our Sun
formed in a cluster some 4.5 billion years ago, and the quest for solar
siblings – stars that were born in the same cluster as the Sun
and then went on different paths – will provide important
information on the birth of our parent star.

- Despite our growing knowledge,
many open questions remain. For instance, how many clusters exist, how
many are currently being formed, how many are falling apart – and
at what pace? The incredible diversity of stars and their birth
clusters is currently being explored by ESA's Gaia satellite.

Figure 40: Gaia's all-sky view
of our Milky Way Galaxy and neighboring galaxies, based on measurements
of nearly 1.7 billion stars and displayed in an equirectangular
projection. It has been obtained by projecting the celestial sphere
onto a rectangle and is suitable for full-dome presentations. The map
shows the total brightness and color of stars observed by the ESA
satellite in each portion of the sky between July 2014 and May 2016.
Brighter regions indicate denser concentrations of especially bright
stars, while darker regions correspond to patches of the sky where
fewer bright stars are observed. The color representation is obtained
by combining the total amount of light with the amount of blue and red
light recorded by Gaia in each patch of the sky (image credit:
ESA/Gaia/DPAC, CC BY-SA 3.0 IGO)

• February 7, 2019:
ESA’s Gaia satellite has looked beyond our Galaxy and explored
two nearby galaxies to reveal the stellar motions within them and how
they will one day interact and collide with the Milky Way – with
surprising results. 55)

- Our Milky Way belongs to a large gathering of galaxies known as the Local Group and, along with the Andromeda and Triangulum galaxies – also referred to as M31 and M33, respectively – makes up the majority of the group’s mass.

- Astronomers have long suspected
that Andromeda will one day collide with the Milky Way, completely
reshaping our cosmic neighborhood. However, the three-dimensional
movements of the Local Group galaxies remained unclear, painting an
uncertain picture of the Milky Way’s future.

- “We needed to explore the
galaxies’ motions in 3D to uncover how they have grown and
evolved, and what creates and influences their features and
behavior,” says lead author Roeland van der Marel of the STScI
(Space Telescope Science Institute) in Baltimore, USA. “We were
able to do this using the second package of high-quality data released
by Gaia.” 56)

- Gaia
is currently building the most precise 3D map of the stars in the
nearby Universe, and is releasing its data in stages. The data from the
second release, made in April 2018, was used in this research.

- Previous studies
of the Local Group have combined observations from telescopes including
the NASA/ESA Hubble Space Telescope and the ground-based Very Long
Baseline Array to figure out how the orbits of Andromeda and Triangulum
have changed over time. The two disc-shaped spiral galaxies are located
between 2.5 and 3 million light-years from us, and are close enough to
one another that they may be interacting.

- Two possibilities emerged: either
Triangulum is on an incredibly long six-billion-year orbit around
Andromeda but has already fallen into it in the past, or it is
currently on its very first infall. Each scenario reflects a different
orbital path, and thus a different formation history and future for
each galaxy.

Figure 41: The future orbital
trajectories of three spiral galaxies: our Milky Way (blue), Andromeda,
also known as M31 (red), and Triangulum, also known as M33 (green). The
circle indicates the current position of each galaxy, and their future
trajectories have been calculated using data from the second release of
ESA’s Gaia mission. The Milky Way is shown as an artist's
impression, while the images of Andromeda and Triangulum are based on
Gaia data. Arrows along the trajectories indicate the estimated
direction of each galaxy's motion and their positions, 2.5 billion
years into the future, while crosses mark their estimated position in
about 4.5 billion years. Approximately 4.5 billion years from now, the
Milky Way and Andromeda will make their first close passage around one
another at a distance of approximately 400,000 light-years. The
galaxies will then continue to move closer to one another and
eventually merge to form an elliptical galaxy (image credit: Orbits: E.
Patel, G. Besla (University of Arizona), R. van der Marel (STScI);
Images: ESA (Milky Way); ESA/Gaia/DPAC (M31, M33)

Figure 42: This gigantic image
of the Triangulum Galaxy — also known as Messier 33 — is a
composite of about 54 different pointings with Hubble’s Advanced
Camera for Surveys. With a staggering size of 34,372 x 19,345 pixels,
it is the second-largest image ever released by Hubble. It is only
dwarfed by the image of the Andromeda Galaxy, released in 2015.
The mosaic of the Triangulum Galaxy showcases the central region of the
galaxy and its inner spiral arms. Millions of stars, hundreds of star
clusters and bright nebulae are visible (image credit: NASA, ESA, and
M. Durbin, J. Dalcanton, and B. F. Williams (University of Washington);
CC BY 4.0)

- While Hubble has obtained the
sharpest view ever of both Andromeda and Triangulum, Gaia measures the
individual position and motion of many of their stars with
unprecedented accuracy.

- “We combed
through the Gaia data to identify thousands of individual stars in both
galaxies, and studied how these stars moved within their galactic
homes,” adds co-author Mark Fardal, also of Space Telescope
Science Institute.

- The stellar motions measured by
Gaia not only reveal how each of the galaxies moves through space, but
also how each rotates around its own spin axis.

- A century ago, when astronomers
were first trying to understand the nature of galaxies, these spin
measurements were much sought-after, but could not be successfully
completed with the telescopes available at the time.

- “It took an observatory as advanced as Gaia to finally do so,” says Roeland.

- “For the first time,
we’ve measured how M31 and M33 rotate on the sky. Astronomers
used to see galaxies as clustered worlds that couldn’t possibly
be separate ‘islands’, but we now know otherwise.

- “It has taken 100 years and
Gaia to finally measure the true, tiny, rotation rate of our nearest
large galactic neighbor, M31. This will help us to understand more
about the nature of galaxies.”

- By combining existing observations
with the new data release from Gaia, the researchers determined how
Andromeda and Triangulum are each moving across the sky, and calculated
the orbital path for each galaxy both backwards and forwards in time
for billions of years.

- “The velocities we found
show that M33 cannot be on a long orbit around M31,” says
co-author Ekta Patel of the University of Arizona, USA. “Our
models unanimously imply that M33 must be on its first infall into
M31.”

- As Andromeda’s motion
differs somewhat from previous estimates, the galaxy is likely to
deliver more of a glancing blow to the Milky Way than a head-on
collision. This will take place not in 3.9 billion years’ time,
but in 4.5 billion – some 600 million years later than
anticipated.

- “We see unusual features in
both M31 and M33, such as warped streams and tails of gas and stars. If
the galaxies haven’t come together before, these can’t have
been created by the forces felt during a merger. Perhaps they formed
via interactions with other galaxies, or by gas dynamics within the
galaxies themselves.”

- “Gaia was designed primarily
for mapping stars within the Milky Way — but this new study shows
that the satellite is exceeding expectations, and can provide unique
insights into the structure and dynamics of galaxies beyond the realm
of our own. The longer Gaia watches the tiny movements of these
galaxies across the sky, the more precise our measurements will
become.”

Figure 43: A
view of the Andromeda galaxy, also known as M31, with measurements of
the motions of stars within the galaxy. This spiral galaxy is the
nearest large neighbor of our Milky Way. The background image, obtained
with NASA's Galex satellite at near-ultraviolet wavelengths, highlights
regions within the galaxy where stars are forming. Blue symbols mark
the locations of bright young stars that were used to measure the
motion of the galaxy, and yellow arrows indicate the average stellar
motions at various locations, based on data from the second release of
ESA’s Gaia satellite. A counter-clockwise rotation of the spiral
galaxy’s disc is evident. The precision of these measurements is
expected to improve with the future Gaia data releases [image credit:
ESA/Gaia (star motions); NASA/Galex (background image); R. van der
Marel, M. Fardal, J. Sahlmann (STScI)]

Legend to Figure 44:
This image, captured with the NASA/ESA Hubble Space Telescope, is the
largest and sharpest image ever taken of the Andromeda galaxy –
otherwise known as M31. This is a cropped version of the full image and
has 1.5 billion pixels. You would need more than 600 HD television
screens to display the whole image. It is the biggest Hubble image ever
released and shows over 100 million stars and thousands of star
clusters embedded in a section of the galaxy's pancake-shaped disc
stretching across over 40,000 light-years.

• January 10, 2019: How old
are each of the stars in our roughly 13-billion-year-old galaxy? A new
technique for understanding the star-forming history of the Milky Way
in unprecedented detail makes it possible to determine the ages of
stars at least two times more precisely than conventional methods,
Embry-Riddle Aeronautical University (ERAU, Daytona Beach, USA)
researchers reported 10 January at the American Astronomical Society
(AAS) meeting in Seattle, WA. 57)

- Current star-dating techniques,
based on assessments of stars in the prime or main sequence of their
lives that have begun to die after exhausting their hydrogen, offer a
20%, or at best a 10% margin of error, explained Embry-Riddle Physics
and Astronomy Professor Dr. Ted von Hippel. Embry-Riddle's approach,
leveraging burnt-out remnants called white dwarf stars, reduces the
margin of error to 5% or even 3%, he said.

- For this method to work, von
Hippel and his team must measure the star's surface temperature,
whether it has a hydrogen or helium atmosphere, and its mass. The
surface temperature can be determined from a star's color and
atmospheric constituents.

- "The star's mass matters because
objects with greater mass have more energy and take longer to cool,"
said von Hippel. "This is why a cup of coffee stays hot longer than a
teaspoon of coffee. Surface temperature, like spent coals in a campfire
that's gone out, offer clues to how long ago the fire died. Finally,
knowing whether there is hydrogen or helium at the surface is important
because helium radiates heat away from the star more readily than
hydrogen."

- Determining the precise masses of
stars, particularly for large samples of white dwarfs, is very
difficult. Now, astronomers have a new method to determine white dwarf
masses.

- Leveraging Gaia Satellite Data: The method takes advantage of data captured by the European Space Agency's Gaia satellite,
an ambitious mission to create a three-dimensional map of the Milky
Way. Von Hippel, with recent Embry-Riddle graduate Adam Moss, current
students Isabelle Kloc, Jimmy Sargent and Natalie Moticksa, and
instructor Elliot Robinson, used highly precise Gaia measurements of
the distance of stars.

- Just as a car's
speedometer may appear to give two different readings from the driver's
perspective versus the passenger's seat, celestial objects can appear
to be in different locations, depending upon the viewer's vantage
point. The Gaia measurements, based on the geometry of two different
lines of site or "parallaxes" to objects, helped Embry-Riddle
researchers determine the radius of stars based on their brightness.
They could then use existing information on the star's mass-to-radius
ratio — a calculation driven by the physical behavior of
electrons — to fill in the last ingredient for determining the
age of the star, its mass.

- Finally, by figuring out the
abundance of different elements within the star, or its metallicity,
researchers can further refine the age of the object, Moss and Kloc
reported in two separate AAS poster presentations. Moss focused on
pairs of stars with one white dwarf and one main sequence star similar
to our Sun, while Kloc's research looked at two white dwarf stars in
the same binary system.

- "The next level of study will be
to determine as many of the elements in the periodic table as possible
for the main sequence star within these pairs," von Hippel said. "That
would tell us more about Galactic chemical evolution, based on how
different elements built up over time as stars formed in our galaxy,
the Milky Way."

- Though he emphasized that the
current work remains preliminary, the team ultimately hopes to publish
the ages of all white dwarf stars within the Gaia dataset: "That could
allow researchers to significantly advance our understanding of
star-formation within the Milky Way."

- Within the field of archaeology,
von Hippel noted, carbon-dating methods made it possible to determine
the age of structures, fossils, Stone Age sites and much more, thereby
providing deeper insights into the evolution of life on Earth. "For
today's astronomers, without knowing the age of different components of
our galaxy, we don't have context. We've had techniques for dating
celestial objects, but not precisely."

- The Embry-Riddle team’s
research collaborators were David Stenning and David van Dyk of
Imperial College London; Elizabeth Jeffery of California Polytechnic
State, San Luis Obispo; Kareem El-Badry of the University of
California, Berkeley; and William Jeffery of the University of Texas,
Austin.

• January 9, 2019: Data
captured by ESA’s galaxy-mapping spacecraft Gaia has revealed for
the first time how white dwarfs, the dead remnants of stars like our
Sun, turn into solid spheres as the hot gas inside them cools down. 58)

- This process of solidification, or
crystallization, of the material inside white dwarfs was predicted 50
years ago but it wasn’t until the arrival of Gaia that
astronomers were able to observe enough of these objects with such a
precision to see the pattern revealing this process.

Figure 45: Illustration of a
white dwarf, the dead remnant of a star like our Sun, with a
crystallized, solid core. Once these stars have burnt all the nuclear
fuel in their core, they shed their outer layers, leaving behind a hot
core that starts cooling down. Data captured by ESA’s
galaxy-mapping spacecraft Gaia has revealed for the first time how
white dwarfs turn into solid spheres as the originally hot matter
inside their core starts crystallizing, becoming solid (image credit:
University of Warwick/Mark Garlick)

- "Previously, we had distances for
only a few hundred of white dwarfs and many of them were in clusters,
where they all have the same age," says Pier-Emmanuel Tremblay from the
University of Warwick, UK, lead author of the paper describing the
results, published today in Nature. 59)

- "With Gaia we now have the
distance, brightness and color of hundreds of thousands of white dwarfs
for a sizeable sample in the outer disc of the Milky Way, spanning a
range of initial masses and all kinds of ages."

- It is in the precise estimate of
the distance to these stars that Gaia makes a breakthrough, allowing
astronomers to gauge their true brightness with unprecedented accuracy.

Figure 46: Stellar evolution:
Artist impression of some possible evolutionary pathways for stars of
different initial masses. Some proto-stars, brown dwarfs, never
actually get hot enough to ignite into fully-fledged stars, and simply
cool off and fade away. Red dwarfs, the most common type of star, keep
burning until they have transformed all their hydrogen into helium,
turning into a white dwarf. Sun-like stars swell into red giants before
puffing away their outer shells into colorful nebula while their cores
collapse into a white dwarf. The most massive stars collapse abruptly
once they have burned through their fuel, triggering a supernova
explosion or gamma-ray burst, and leaving behind a neutron star or
black hole (image credit: ESA)

- White dwarfs are the remains of
medium-sized stars similar to our Sun. Once these stars have burnt all
the nuclear fuel in their core, they shed their outer layers, leaving
behind a hot core that starts cooling down.

- These ultra-dense remnants still
emit thermal radiation as they cool, and are visible to astronomers as
rather faint objects. It is estimated that up to 97% of stars in the
Milky Way will eventually turn into white dwarfs, while the most
massive of stars will end up as neutron stars or black holes.

- The cooling of white dwarfs lasts
billions of years. Once they reach a certain temperature, the
originally hot matter inside the star’s core starts
crystallizing, becoming solid. The process is similar to liquid water
turning into ice on Earth at zero degrees Celsius, except that the
temperature at which this solidification happens in white dwarfs is
extremely high – about 10 million degrees Celsius.

- In this study, the astronomers
analyzed more than 15,000 stellar remnant candidates within 300 light
years of Earth as observed by Gaia and were able to see these
crystallizing white dwarfs as a rather distinct group.

Figure 47: This diagram, known
as Hertzsprung-Russell diagram (after the astronomers who devised it in
the early 20th century to study stellar evolution) combines information
about the brightness, color and distance of more than 15,000 white
dwarfs within 300 light years of Earth. The data, shown as black dots,
are from the second release of ESA’s Gaia satellite (image
credit: Pier-Emmanuel Tremblay, et al.)

- “We saw a pile-up of white
dwarfs of certain colors and luminosities that were otherwise not
linked together in terms of their evolution,” says Pier-Emmanuel.
“We realized that this was not a distinct population of white
dwarfs, but the effect of the cooling and crystallization predicted 50
years ago.”

- The heat released during this
crystallization process, which lasts several billion years, seemingly
slows down the evolution of the white dwarfs: the dead stars stop
dimming and, as a result, appear up to two billion years younger than
they actually are. That, in turn, has an impact on our understanding of
the stellar groupings these white dwarfs are a part of.

- White dwarfs are
traditionally used for age-dating of stellar populations such as
clusters of stars, the outer disc, and the halo in our Milky
Way,” explains Pier-Emmanuel. “We will now have to develop
better crystallization models to get more accurate estimates of the
ages of these systems.”

- Not all white dwarfs crystallize
at the same pace. More massive stars cool down more rapidly and will
reach the temperature at which crystallization happens in about one
billion years. White dwarfs with lower masses, closer to the expected
end stage of the Sun, cool in a slower fashion, requiring up to six
billion years to turn into dead solid spheres.

- The Sun still has about five
billion years before it becomes a white dwarf, and the astronomers
estimate that it will take another five billion years after that to
eventually cool down to a crystal sphere.

- “This result highlights the
versatility of Gaia and its numerous applications,” says Timo
Prusti, Gaia project scientist at ESA. “It’s exciting how
scanning stars across the sky and measuring their properties can lead
to evidence of plasma phenomena in matter so dense that cannot be
tested in the laboratory.”

• December 13, 2018: Launched
in December 2013, ESA’s Gaia satellite has been scanning the sky
to perform the most precise stellar census of our Milky Way galaxy,
observing more than one billion stars and measuring their positions,
distances and motions to unprecedented accuracy. — Launched in
December 2013, ESA’s Gaia satellite has been scanning the sky to
perform the most precise stellar census of our Milky Way galaxy,
observing more than one billion stars and measuring their positions,
distances and motions to unprecedented accuracy. 60)

• November 14, 2018:
The SPC (Science Program Committee) of ESA has confirmed the continued
operations of ten scientific missions in the Agency's fleet up to 2022.
After a comprehensive review of their scientific merits and technical
status, the SPC has decided to extend the operation of the five
missions led by ESA's Science Program: Cluster, Gaia, INTEGRAL, Mars Express, and XMM-Newton. The SPC also confirmed the Agency's contributions to the extended operations of Hinode, Hubble, IRIS, SOHO, and ExoMars TGO. 61)

- This includes the
confirmation of operations for the 2019–2020 cycle for missions
that had been given indicative extensions as part of the previous
extension process, and indicative extensions for an additional two
years, up to 2022.
Note: Every two years, all missions whose approved operations end
within the following four years are subject to review by the advisory
structure of the Science Directorate. Extensions are granted to
missions that satisfy the established criteria for operational status
and science return, subject to the level of financial resources
available in the science program. These extensions are valid for the
following four years, subject to a mid-term review and confirmation
after two years.

- The decision was taken
during the SPC meeting at ESA/ESAC (European Space Astronomy Center)
near Madrid, Spain, on 14 November.

- ESA's science missions have
unique capabilities and are prolific in their scientific output.
Cluster, for example, is the only mission that, by varying the
separation between its four spacecraft, allows multipoint measurements
of the magnetosphere in different regions and at different scales,
while Gaia is performing the most precise astrometric survey ever
realized, enabling unprecedented studies of the distribution and
motions of stars in the Milky Way and beyond.

- Many of the science missions
are proving to be of great value to pursue investigations that were not
foreseen at the time of their launch. Examples include the role of
INTEGRAL and XMM-Newton in the follow-up of recent gravitational wave
detections, paving the way for the future of multi-messenger astronomy,
and the many discoveries of diverse exoplanets by Hubble.

- Collaboration between
missions, including those led by partner agencies, is also of great
importance. The interplay between solar missions like Hinode, IRIS and
SOHO provides an extensive suite of complementary instruments to study
our Sun; meanwhile, Mars Express and ExoMars TGO are at the forefront
of the international fleet investigating the Red Planet.

- Another compelling factor to
support the extension is the introduction of new modes of operation to
accommodate the evolving needs of the scientific community, as well as
new opportunities for scientists to get involved with the missions.

• October 31, 2018: ESA's Gaia
mission has made a major breakthrough in unravelling the formation
history of the Milky Way. Instead of forming alone, our Galaxy merged
with another large galaxy early in its life, around 10 billion years
ago. The evidence is littered across the sky all around us, but it has
taken Gaia and its extraordinary precision to show us what has been
hiding in plain sight all along. 62)63)

Legend to Figure 49:
Astronomers uncovered this major event in the formation history of the
Milky Way after discovering an ‘odd collection’ of stars
that move along elongated trajectories in the opposite direction to the
majority of the Galaxy’s other hundred billion stars, including
the Sun. The discovery was possible thanks to the second data release
of ESA’s Gaia mission and its extraordinary precision. The
positions and motions of the stars in Gaia-Enceladus (represented with
yellow arrows) in this early phase of the merger are based on a
computer simulation that models a similar encounter to that uncovered
by Gaia.

- Using the first 22 months of observations,
a team of astronomers led by Amina Helmi, University of Groningen, The
Netherlands, looked at seven million stars – those for which the
full 3D positions and velocities are available – and found that
some 30,000 of them were part of an ‘odd collection’ moving
through the Milky Way. The observed stars in particular are currently
passing by our solar neighborhood.

- We are so deeply
embedded in this collection that its stars surround us almost
completely, and so can be seen across most of the sky.

Figure 50: Debris of galactic
merger: Artist’s impression of debris of the Gaia-Enceladus
galaxy. Gaia-Enceladus merged with our Milky Way galaxy during its
early formation stages, 10 billion years ago, and its debris can now be
found throughout the Galaxy (image credit: ESA (artist’s
impression and composition); Koppelman, Villalobos and Helmi
(simulation), CC BY-SA 3.0 IGO) 64)

- Even though they are interspersed with other stars, the stars in the collection stood out in the Gaia data
because they all move along elongated trajectories in the opposite
direction to the majority of the Galaxy’s other hundred billion
stars, including the Sun.

- They also stood out in the so-called Hertzprung-Russell diagram
– which is used to compare the color and brightness of stars
– indicating that they belong to a clearly distinct stellar
population.

- The sheer number of odd-moving
stars involved intrigued Amina and her colleagues, who suspected they
might have something to do with the Milky Way’s formation history
and set to work to understand their origins.

- In the past, Amina and her
research group had used computer simulations to study what happens to
stars when two large galaxies merge. When she compared those to the
Gaia data, the simulated results matched the observations.

- “The collection of stars we
found with Gaia has all the properties of what you would expect from
the debris of a galactic merger,” says Amina, lead author of the
paper published today in Nature. 65)

- In other words,
the collection is what they expected from stars that were once part of
another galaxy and have been consumed by the Milky Way. The stars now
form most of our Galaxy’s inner halo – a diffuse component
of old stars that were born at early times and now surround the main
bulk of the Milky Way known as the central bulge and disc.

Legend to Figure 51:
On the left, a face-on view shows the spiral structure of the Galactic
Disc, where the majority of stars are located, interspersed with a
diffuse mixture of gas and cosmic dust. The disc measures about 100,000
light-years across, and the Sun sits about half way between its center
and periphery. - On the right, an edge-on view reveals the flattened
shape of the disc. Observations point to a substructure: a thin disc
some 700 light-years high embedded in a thick disc, about 3000
light-years high and populated with older stars.
The edge on view also shows the Galactic Bulge, located in the central
portion of the Milky Way and hosting about 10 billion stars, which are
mainly old and red. The bulge, also visible in the face-on view on the
left, has an overall elongated shape that resembles that of a
peanut-shaped bar, with a half-length of about 10,000 light-years,
making the Milky Way a barred spiral galaxy.
Beyond the disc and bulge is the stellar halo, a roughly spherical
structure with a radius of about 100 000 light-years, containing
isolated stars as well as many globular clusters – large, compact
conglomerations of some of the most ancient stars in the Galaxy. On a
grander scale, the Milky Way is embedded in an even larger halo of
invisible dark matter.

- The Galactic disc itself is
composed of two parts. There is the thin disc, which is a few hundred
light years deep and contains the pattern of spiral arms made by bright
stars. And there is the thick disc, which is a few thousand light years
deep. It contains about 10–20 percent of the Galaxy's stars yet
its origins have been difficult to determine.

- According to the
team's simulations, as well as supplying the halo stars, the accreted
galaxy could also have disturbed the Milky Way's pre-existing stars to
help form the thick disc.

- "We became only certain about our
interpretation after complementing the Gaia data with additional
information about the chemical composition of stars, supplied by the
ground-based APOGEE survey," says Carine Babusiaux, Université
Grenoble Alpes, France, and second author of the paper.

- Stars that form in different
galaxies have unique chemical compositions that match the conditions of
the home galaxy. If this star collection was indeed the remains of a
galaxy that merged with our own, the stars should show an imprint of
this in their composition. And they did.

- The astronomers called this galaxy
Gaia-Enceladus after one of the Giants in ancient Greek mythology,
­who was the offspring of Gaia, the Earth, and Uranus, the Sky.

- "According to the legend,
Enceladus was buried under Mount Etna, in Sicily, and responsible for
local earthquakes. Similarly, the stars of Gaia-Enceladus were deeply
buried in the Gaia data, and they have shaken the Milky Way, leading to
the formation of its thick disc," explains Amina.

- Even though no more evidence was
really needed, the team also found hundreds of variable stars and 13
globular clusters in the Milky Way that follow similar trajectories as
the stars from Gaia-Enceladus, indicating that they were originally
part of that system. Globular clusters are groups of up to millions of
stars, held together by their mutual gravity and orbiting the center of
a galaxy. The fact that so many clusters could be linked to
Gaia-Enceladus is another indication that this must have once been a
big galaxy in its own right, with its own entourage of globular
clusters.

Legend to Figure 52:
All-sky distribution of an 'odd collection' of stars detected in the
second data release of ESA's Gaia mission. These stars move along
elongated trajectories in the opposite direction to the majority of our
Milky Way's other hundred billion stars and have a markedly different
chemical composition, indicating that they belong to a clearly distinct
stellar population.
From these clues, astronomers inferred that these stars are the debris
of a galaxy that merged with our Milky Way during its early formation
stages, 10 billion years ago, and named this galaxy Gaia-Enceladus.
The stars of Gaia-Enceladus are represented with different colors
depending on their parallax – a measure of their distance –
with purple hues indicating the most nearby stars and yellow hues the
most distant ones. White circles indicate globular clusters that were
observed to follow similar trajectories as the stars from
Gaia-Enceladus, indicating that they were originally part of that
system; cyan star symbols indicate variable stars that are also
associated as Gaia-Enceladus debris.

- Further analysis
revealed that this galaxy was about the size of one of the Magellanic
Clouds – two satellite galaxies roughly ten times smaller than
the current size of the Milky Way.

- Ten billion years ago, however,
when the merger with Gaia-Enceladus took place, the Milky Way itself
was much smaller, so the ratio between the two was more like four to
one. It was therefore clearly a major blow to our Galaxy.

- "Seeing that we are now starting
to unravel the formation history of the Milky Way is very exciting,"
says Anthony Brown, Leiden University, The Netherlands, who is a
co-author of the paper and also chair of the Gaia Data Processing and
Analysis Consortium Executive.

- Since the very first discussions
about building Gaia 25 years ago, one of the mission's key objectives
was to examine the various stellar streams in the Milky Way, and
reconstruct its early history. That vision is paying off.

- "Gaia was built to answer such
questions," says Amina. "We can now say this is the way the Galaxy
formed in those early epochs. It's fantastic. It's just so beautiful
and makes you feel so big and so small at the same time."

- "By reading the motions of stars
scattered across the sky, we are now able to rewind the history of the
Milky Way and discover a major milestone in its formation, and this is
possible thanks to Gaia," concludes Timo Prusti, Gaia project scientist
at ESA.

• October 02, 2018: A team of
astronomers using the latest set of data from ESA's Gaia mission to
look for high-velocity stars being kicked out of the Milky Way were
surprised to find stars instead sprinting inwards – perhaps from
another galaxy. 66)

Figure 53: The positions and
reconstructed orbits of 20 high-velocity stars, represented on top of
an artistic view of our Galaxy, the Milky Way. These stars were
identified using data from the second release of ESA’s Gaia mission.
The seven stars shown in red are sprinting away from the Galaxy and
could be travelling fast enough to eventually escape its gravity.
Surprisingly, the study revealed also thirteen stars, shown in orange,
that are racing towards the Milky Way: these could be stars from
another galaxy, zooming right through our own [image credit: ESA
(artist’s impression and composition); Marchetti et al 2018 (star
positions and trajectories); NASA/ESA/Hubble (background galaxies), CC
BY-SA 3.0 IGO]

- In April, ESA’s stellar
surveyor Gaia released an unprecedented catalogue of more than one
billion stars. Astronomers across the world have been working
ceaselessly over the past few months to explore this extraordinary
dataset, scrutinizing the properties and motions of stars in our Galaxy
and beyond with never before achieved precision, giving rise to a
multitude of new and intriguing studies.

- The Milky Way contains over a
hundred billion stars. Most are located in a disk with a dense, bulging
center, at the middle of which is a supermassive black hole. The rest
are spread out in a much larger spherical halo.

- Stars circle around the Milky Way
at hundreds of km/s, and their motions contain a wealth of information
about the past history of the Galaxy. The fastest class of stars in our
Galaxy are called hypervelocity stars, which are thought to start their
life near the Galactic center to be later flung towards the edge of the
Milky Way via interactions with the black hole.

- Only a small number of
hypervelocity stars have ever been discovered, and Gaia’s
recently published second data release provides a unique opportunity to
look for more of them.

- Several groups of astronomers
jumped into the brand-new dataset in search of hypervelocity stars
immediately after the release. Among them, three scientists at Leiden
University, the Netherlands, were in for a big surprise.

- For 1.3 billion
stars, Gaia measured positions, parallaxes – an indicator of
their distance – and 2D motions on the plane of the sky. For
seven million of the brightest ones, it also measured how quickly they
move towards or away from us.

- “Of the seven million Gaia
stars with full 3D velocity measurements, we found twenty that could be
travelling fast enough to eventually escape from the Milky Way,”
explains Elena Maria Rossi, one of the authors of the new study. 67)

- “Rather than flying away
from the Galactic center, most of the high velocity stars we spotted
seem to be racing towards it,” adds co-author Tommaso Marchetti.
— “These could be stars from another galaxy, zooming right
through the Milky Way.”

- It is possible that these
intergalactic interlopers come from the Large Magellanic Cloud, a
relatively small galaxy orbiting the Milky Way, or they may originate
from a galaxy even further afield.

Figure 54:
The Large Magellanic Cloud (LMC), one of the nearest galaxies to our
Milky Way, as viewed by ESA’s Gaia satellite using information
from the mission’s second data release. This view is not a
photograph but has been compiled by mapping the total amount of
radiation detected by Gaia in each pixel, combined with measurements of
the radiation taken through different filters on the spacecraft to
generate color information [image credit: ESA/Gaia/DPAC (Data
Processing and Analysis Consortium); A. Moitinho / A. F. Silva / M.
Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork
Research, Portugal]

Legend to Figure 54:
The image is dominated by the brightest, most massive stars, which
greatly outshine their fainter, lower-mass counterparts. In this view,
the bar of the LMC is outlined in great detail, along with individual
regions of star formation like the giant 30 Doradus, visible just above
the center of the galaxy.

- If that is the case, they carry
the imprint of their site of origin, and studying them at much closer
distances than their parent galaxy could provide unprecedented
information on the nature of stars in another galaxy – similar in
a way to studying martian material brought to our planet by meteorites.

- “Stars can be accelerated to
high velocities when they interact with a supermassive black
hole,” Elena explains. “So the presence of these stars
might be a sign of such black holes in nearby galaxies. But the stars
may also have once been part of a binary system, flung towards the
Milky Way when their companion star exploded as a supernova. Either
way, studying them could tell us more about these kinds of processes in
nearby galaxies.”

- An alternative explanation is that
the newly identified sprinting stars could be native to our
Galaxy’s halo, accelerated and pushed inwards through
interactions with one of the dwarf galaxies that fell towards the Milky
Way during its build-up history. Additional information about the age
and composition of the stars could help the astronomers clarify their
origin.

- “A star from the Milky Way
halo is likely to be fairly old and mostly made of hydrogen, whereas
stars from other galaxies could contain lots of heavier
elements,” says Tommaso. - “Looking at the colors of stars
tells us more about what they are made of.”

- New data will help nail down the
nature and origin of these stars with more certainty, and the team will
use ground-based telescopes to find out more about them. In the
meantime, Gaia continues to make observations of the full sky,
including the stars analyzed in this study.

- While investigating the nature of
these possible stellar interlopers, the team is also busy digging into
the full dataset from Gaia’s second release, searching for more
high-speed stars and looking forward to the future. At least two more Gaia data releases are planned in the 2020s, and each will provide both more precise and new information on a larger set of stars.

- “We eventually expect full
3D velocity measurements for up to 150 million stars,” explains
co-author Anthony Brown, chair of the Gaia Data Processing and Analysis
Consortium Executive. “This will help find hundreds or thousands
of hypervelocity stars, understand their origin in much more detail,
and use them to investigate the Galactic center environment as well as
the history of our Galaxy,” he adds.

• September 25, 2018: Using
data from ESA’s Gaia stellar surveyor, astronomers have
identified four stars that are possible places of origin of
‘Oumuamua, an interstellar object spotted during a brief visit to
our Solar System in 2017. 68)

- The discovery last year sparked a large observational campaign: originally identified as the first known interstellar asteroid, the small body was later revealed to be a comet,
as further observations showed it was not slowing down as fast as it
should have under gravity alone. The most likely explanation of the
tiny variations recorded in its trajectory was that they are caused by
gasses emanating from its surface, making it more akin to a comet.

- But where in the Milky Way did this cosmic traveller come from?

- Comets are leftovers of the
formation of planetary systems, and it is possible that ‘Oumuamua
was ejected from its home star’s realm while planets were still
taking shape there. To look for its home, astronomers had to trace back
in time not only the trajectory of the interstellar comet, but also of
a selection of stars that might have crossed paths with this object in
the past few million years.

- “Gaia is a powerful time
machine for these types of studies, as it provides not only star
positions but also their motions,” explains Timo Prusti, Gaia
project scientist at ESA.

- To this aim, a team of astronomers
led by Coryn Bailer-Jones at the Max Planck Institute for Astronomy in
Heidelberg, Germany, dived into the data from Gaia’s second release, which was made public in April. 69)

- The Gaia data contain positions,
distance indicators and motions on the sky for more than a billion
stars in our Galaxy; most importantly, the data set includes radial
velocities – how fast they are moving towards or away from us
– for a subset of seven million, enabling a full reconstruction
of their trajectories. The team looked at these seven million stars,
complemented with an extra 220,000 for which radial velocities are
available from the astronomical literature.

- As a result,
Coryn and colleagues identified four stars whose orbits had come within
a couple of light years of ‘Oumuamua in the near past, and with
relative velocities low enough to be compatible with likely ejection
mechanisms.

- All four are dwarf stars – with masses similar to or smaller than our Sun’s – and had their ‘close’ encounter with the interstellar comet
between one and seven million years ago. However, none of them is known
to either harbor planets or to be part of a binary stellar system; a
giant planet or companion star would be the preferred mechanism to have
ejected the small body.

- While future observations of these
four stars might shed new light on their properties and potential to be
the home system of ‘Oumuamua, the astronomers are also looking
forward to future releases of Gaia data. At least two are planned in
the 2020s, which will include a much larger sample of radial
velocities, enabling them to reconstruct and investigate the
trajectories of many more stars.

- “While it’s still
early to pinpoint ‘Oumuamua’s home star, this result
illustrates the power of Gaia to delve into the history of our Milky
Way galaxy,” concludes Timo.

Figure 55: Artist impression of
the interstellar object ‘Oumuamua. Observations since its
discovery in 2017 show that the object is slightly deviating from the
trajectory it would be following if it were only influenced by the
gravity of the Sun and the planets. Researchers assume that venting
material from its surface due to solar heating is responsible for this
behavior. This outgassing can be seen in this artist’s impression
as a subtle cloud being ejected from the side of the object facing the
Sun (image credit: ESA/Hubble, NASA, ESO, M. Kornmesser)

• September 19, 2018:
ESA’s star mapping mission, Gaia, has shown our Milky Way galaxy
is still enduring the effects of a near collision that set millions of
stars moving like ripples on a pond. The close encounter likely took
place sometime in the past 300–900 million years. It was
discovered because of the pattern of movement it has given to stars in
the Milky Way disk – one of the major components of our Galaxy. 70)

- The pattern was
revealed because Gaia not only accurately measures the positions of
more than a billion stars but also precisely measures their velocities
on the plane of the sky. For a subset of a few million stars, Gaia
provided an estimate of the full three-dimensional velocities, allowing
a study of stellar motion using the combination of position and
velocity, which is known as ‘phase space’.

Figure 56:
Artist’s impression of a perturbation in the velocities of stars
in our Galaxy, the Milky Way, that was revealed by ESA’s star
mapping mission, Gaia. Scientists analyzing data from Gaia’s
second release have shown our Milky Way galaxy is still enduring the
effects of a near collision that set millions of stars moving like
ripples on a pond (image credit: ESA, CC BY-SA 3.0 IGO)

- In phase space, the stellar
motions revealed an interesting and totally unexpected pattern when the
star’s positions were plotted against their velocities. Teresa
Antoja from Universitat de Barcelona, Spain, who led the research
couldn’t quite believe her eyes when she first saw it on her
computer screen.

- One shape in
particular caught her attention. It was a snail shell-like pattern in
the graph that plotted the stars’ altitude above or below the
plane of the Galaxy against their velocity in the same direction. It
had never been seen before. “At the beginning the features were
very weird to us,” says Teresa. “I was a bit shocked and I
thought there could be a problem with the data because the shapes are
so clear.”

Figure 57:
Snail shell pattern in the velocity of stars. This graph shows the
altitude of stars in our Galaxy above or below the plane of the Milky
Way against their velocity in the same direction, based on a simulation
of a near collision that set millions of stars moving like ripples on a
pond (image credit: T. Antoja et al. 2018)

Legend to Figure 57:
The snail shell-like shape of the pattern reproduces a feature that was
first seen in the movement of stars in the Milky Way disk using data
from the second release of ESA’s Gaia mission, and interpreted as
an imprint of a galactic encounter. - The close encounter revealed by
the Gaia data likely took place sometime in the past 300–900
million years, and the culprit could be the Sagittarius dwarf galaxy, a
small galaxy containing a few tens of millions of stars that is
currently in the process of being cannibalized by the Milky Way.

- But the Gaia data had undergone
multiple validation tests by the Gaia Data Processing and Analysis
Consortium teams all over Europe before release. Also, together with
collaborators, Teresa had performed many tests on the data to look for
errors that could be forcing such shapes on the data. Yet no matter
what they checked, the only conclusion they could draw was that these
features do indeed exist in reality.

- The reason they had not been seen
before was because the quality of the Gaia data was a huge step up from
what had come before. “It looks like suddenly you have put the
right glasses on and you see all the things that were not possible to
see before,” says Teresa.

- With the reality of the structure
confirmed, it came time to investigate why it was there. “It is a
bit like throwing a stone in a pond, which displaces the water as
ripples and waves,” explains Teresa.

- Unlike the water molecules, which
settle again, the stars retain a ‘memory’ that they were
perturbed. This memory is found in their motions. After some time,
although the ripples may no longer be easily visible in the
distribution of stars, they are still there when you look in their
velocities.

- The researchers looked up previous
studies that had investigated such ‘phase mixing’ in other
astrophysical settings and in quantum physics situations. Although no
one had investigated this happening in the disk of our Galaxy, the
structures were clearly reminiscent of each other.

- “I find this really amazing
that we can see this snail shell shape. It is just like it appears in
text books,” says Amina Helmi, University of Groningen, The
Netherlands, a collaborator on the project and the second author on the
resulting paper. 71)

- So the next question was what had
‘hit’ the Milky Way to cause this behavior in the stars. We
know that our Galaxy is a cannibal. It grows by eating smaller galaxies
and clusters of stars that then mix in with the rest of the Galaxy. But
that didn’t seem to be the case here.

- Then Amina recalled her own and
others studies of the Sagittarius dwarf galaxy. This small galaxy
contains a few tens of millions of stars and is currently in the
process of being cannibalized by the Milky Way.

- Its last close pass to our Galaxy
was not a direct hit – it passed close by. This would have been
enough so that its gravity perturbed some stars in our Galaxy like a
stone dropping into water.

- The clincher was that estimates of
Sagittarius’s last close encounter with the Milky Way place it
sometime between 200 and 1000 million years ago, which is almost
exactly what Teresa and colleagues calculated as an origin for the
beginning of the snail shell-like pattern.

- So far, however, the association
of the snail shell feature with the Sagittarius dwarf galaxy is based
on simple computer models and analyses. The next step is to scrutinize
the phenomenon more fully to gain knowledge of the Milky Way.

- The scientists plan to investigate
this galactic encounter as well as the distribution of matter in the
Milky Way by using the information contained in the snail shell shape.
One thing is certain. There is a lot of work to do.

- “The discovery was easy; the
interpretations harder. And the full understanding of its meaning and
implications might take several years.” said Amina.

- Gaia is one of ESA’s
cornerstone missions and was designed primarily to investigate the
origin, evolution and structure of the Milky Way. In April, it made
available its second data release, which is the data that made this discovery possible.

- “This is exactly the kind of
discovery we hoped would come from the Gaia data,” says Timo
Prusti, Gaia project scientist at ESA. “The Milky Way has a rich
history to tell, and we are starting to read that story.”

Figure 58: The Sagittarius dwarf
galaxy in Gaia's all-sky view. The Sagittarius dwarf galaxy, a small
satellite of the Milky Way that is leaving a stream of stars behind as
an effect of our Galaxy’s gravitational tug, is visible as an
elongated feature below the Galactic center and pointing in the
downwards direction in the all-sky map of the density of stars observed
by ESA’s Gaia mission between July 2014 to May 2016 (image
credit: ESA/Gaia/DPAC)

• August 20,2018: The mass of
a very young exoplanet has been revealed for the first time using data
from ESA’s star mapping spacecraft Gaia and its predecessor, the
quarter-century retired Hipparcos satellite. 72)

- Astronomers Ignas Snellen and
Anthony Brown from Leiden University, the Netherlands, deduced the mass
of the planet Beta Pictoris b from the motion of its host star over a
long period of time as captured by both Gaia and Hipparcos.

- The planet is a gas giant similar
to Jupiter but, according to the new estimate, is 9 to 13 times more
massive. It orbits the star Beta Pictoris, the second brightest star in
the constellation Pictor.

Figure 59: The planet Beta
Pictoris b is visible orbiting its host star in this composite image
from the European Southern Observatory’s (ESO) 3.6 m telescope
and the NACO (Nasmyth Adaptive+CONICA Optics) instrument on ESO’s
8.2-m VLT (Very Large Telescope). The Beta Pictoris system is only
about 20 million years old, roughly 225 times younger than the Solar
System. Observing this dynamic and rapidly evolving system can help
astronomers shed light on the processes of planet formation and early
evolution (image credit: ESO/A-M. Lagrange et al.)

- The planet was only discovered in
2008 in images captured by the VLT (Very Large Telescope) at the
European Southern Observatory in Chile. Both the planet and the star
are only about 20 million years old – roughly 225 times younger
than the Solar System. Its young age makes the system intriguing but
also difficult to study using conventional methods.

- “In the Beta Pictoris
system, the planet has essentially just formed,” says Ignas.
“Therefore we can get a picture of how planets form and how they
behave in the early stages of their evolution. On the other hand, the
star is very hot, rotates fast, and it pulsates.”

- This behavior makes it difficult
for astronomers to accurately measure the star’s radial velocity
– the speed at which it appears to periodically move towards and
away from the Earth. Tiny changes in the radial velocity of a star,
caused by the gravitational pull of planets in its vicinity, are
commonly used to estimate masses of exoplanets. But this method mainly
works for systems that have already gone through the fiery early stages
of their evolution.

- In the case of Beta Pictoris b,
upper limits of the planet’s mass range had been arrived at
before using the radial velocity method. To obtain a better estimate,
the astronomers used a different method, taking advantage of
Hipparcos’ and Gaia’s measurements that reveal the precise
position and motion of the planet’s host star in the sky over
time.

Figure 60: Astronomers can
measure the mass of exoplanets by looking at tiny deviations in the
trajectories of their host stars caused by the gravitational pull of
the orbiting planets. These can be observed either along the line of
sight, looking for small changes in a star’s radial velocity, or
on the plane of the sky, using astrometric measurements. To be able to
make accurate assessments, the astrometric observations need to cover a
period of many years. In this picture, the white dashed spiral shows
the evolution of a star’s trajectory observable from the Earth,
caused by the combination of parallax and proper motion. The brown band
shows the range of deviations of the star’s trajectory caused by
a possible planet orbiting it (image credit: ESA) 73)

- “The star moves for
different reasons,” says Ignas. “First, the star circles
around the center of the Milky Way, just as the Sun does. That appears
from the Earth as a linear motion projected on the sky. We call it
proper motion. And then there is the parallax effect, which is caused
by the Earth orbiting around the Sun. Because of this, over the year,
we see the star from slightly different angles.”

- And then there is something that
the astronomers describe as ‘tiny wobbles’ in the
trajectory of the star across the sky – minuscule deviations from
the expected course caused by the gravitational pull of the planet in
the star’s orbit. This is the same wobble that can be measured
via changes in the radial velocity, but along a different direction
– on the plane of the sky, rather than along the line of sight.

- “We are looking at the
deviation from what you expect if there was no planet and then we
measure the mass of the planet from the significance of this
deviation,” says Anthony. “The more massive the planet, the
more significant the deviation.”

- To be able to make such an
assessment, astronomers need to observe the trajectory of the star for
a long period of time to properly understand the proper motion and the
parallax effect.

- The Gaia
mission, designed to observe more than one billion stars in our Galaxy,
will eventually be able to provide information about a large amount of
exoplanets. In the 22 months of observations included in Gaia’s
second data release, published in April, the satellite has recorded the
star Beta Pictoris about thirty times. That, however, is not enough.

- “Gaia will find thousands of
exoplanets, that’s still on our to-do list,” says Timo
Prusti, ESA’s Gaia project scientist. “The reason that the
exoplanets can be expected only late in the mission is the fact that to
measure the tiny wobble that the exoplanets are causing, we need to
trace the position of stars for several years.”

- Combining the Gaia measurements
with those from ESA’s Hipparcos mission, which observed Beta
Pictoris 111 times between 1990 and 1993, enabled Ignas and Anthony to
get their result much faster. This led to the first successful estimate
of a young planet’s mass using astrometric measurements.

- “By combining data from
Hipparcos and Gaia, which have a time difference of about 25 years, you
get a very long term proper motion,” says Anthony. “This
proper motion also contains the component caused by the orbiting
planet. Hipparcos on its own would not have been able to find this
planet because it would look like a perfectly normal single star unless
we had measured it for a much longer time. Now, by combining Gaia and
Hipparcos and looking at the difference in the long term and the short
term proper motion, we can see the effect of the planet on the
star.”

- The result represents an important
step towards better understanding the processes involved in planet
formation, and anticipates the exciting exoplanet discoveries that will
be unleashed by Gaia’s future data releases. 74)

• August 20, 2018: The second
data release of ESA’s Gaia mission, made in April, has marked a
turning point in the study of our Galactic home, the Milky Way. With an
unprecedented catalog of 3D positions and 2D motions of more than a
billion stars, plus additional information on smaller subsets of stars
and other celestial sources, Gaia has provided astronomers with an
astonishing resource to explore the distribution and composition of the
Galaxy and to investigate its past and future evolution. 75)

- The majority of stars in the Milky
Way are located in the Galactic disc, which has a flattened shape
characterized by a pattern of spiral arms similar to that observed in
spiral galaxies beyond our own. However, it is particularly challenging
to reconstruct the distribution of stars in the disc, and especially
the design of the Milky Way’s arms, because of our position
within the disc itself. — This is where Gaia’s measurements
can make the difference.

- The image of Figure 61
shows a 3D map obtained by focusing on one particular type of object:
OB stars, the hottest, brightest and most massive stars in our Galaxy.
Because these stars have relatively short lives – up to a few
tens of million years – they are mostly found close to their
formation sites in the Galactic disc. As such, they can be used to
trace the overall distribution of young stars, star formation sites,
and the Galaxy’s spiral arms.

- The map, based on 400,000 of this
type of star within less than 10,000 light-years from the Sun, was
created by Kevin Jardine, a software developer and amateur astronomer
with an interest in mapping the Milky Way using a variety of
astronomical data.

- It is centered on the Sun and
shows the Galactic disc as if we were looking at it face-on from a
vantage point outside the Galaxy.

- To deal with the massive number of
stars in the Gaia catalog, Kevin made use of so-called density
isosurfaces, a technique that is routinely used in many practical
applications, for example to visualize the tissue of organs of bones in
CT (Computer Tomography) scans of the human body. In this technique,
the 3D distribution of individual points is represented in terms of one
or more smooth surfaces that delimit regions with a different density
of points.

- Here, regions of the Galactic disc
are shown with different colors depending on the density of ionizing
stars recorded by Gaia; these are the hottest among OB stars, shining
with ultraviolet radiation that knocks electrons off hydrogen atoms to
give them their ionized state.

- The regions with the highest
density of these stars are displayed in pink/purple shades, regions
with intermediate density in violet/light blue, and low-density regions
in dark blue. Additional information from other astronomical surveys
was also used to map concentrations of interstellar dust, shown in
green, while known clouds of ionized gas are depicted as red spheres.

- The appearance of
‘spokes’ is a combination of dust clouds blocking the view
to stars behind them and a stretching effect of the distribution of
stars along the line of sight.

- An interactive version of this map is also available as part of Gaia Sky,
a real-time, 3D astronomy visualization software that was developed in
the framework of the Gaia mission at the Astronomisches
Rechen-Institut, University of Heidelberg, Germany.

• April 25, 2018: Gaia's second release of the star catalog.ESA’s
Gaia mission has produced the richest star catalog to date, including
high-precision measurements of nearly 1.7 billion stars and revealing
previously unseen details of our home Galaxy. A multitude of
discoveries are on the horizon after this much awaited release, which
is based on 22 months of charting the sky. The new data includes
positions, distance indicators and motions of more than one billion
stars, along with high-precision measurements of asteroids within our
Solar System and stars beyond our own Milky Way Galaxy.76)77)

Note 1: A series of scientific
papers describing the data contained in the release and their
validation process will appear in a special issue of Astronomy & Astrophysics.

Note 2: A series of 360-degree videos and other Virtual Reality visualization resources are available at http://sci.esa.int/gaia-vr

- Preliminary analysis of this
phenomenal data reveals fine details about the make-up of the Milky
Way’s stellar population and about how stars move, essential
information for investigating the formation and evolution of our home
Galaxy. -“The observations collected by Gaia are redefining the
foundations of astronomy,” says Günther Hasinger, ESA
Director of Science.

Legend to Figure 62:
Brighter regions indicate denser concentrations of especially bright
stars, while darker regions correspond to patches of the sky where
fewer bright stars are observed. The color representation is obtained
by combining the total amount of light with the amount of blue and red
light recorded by Gaia in each patch of the sky.

The bright horizontal structure that
dominates the image is the Galactic plane, the flattened disc that
hosts most of the stars in our home Galaxy. In the middle of the image,
the Galactic center appears vivid and teeming with stars. Darker
regions across the Galactic plane correspond to foreground clouds of
interstellar gas and dust, which absorb the light of stars located
further away, behind the clouds. Many of these conceal stellar
nurseries where new generations of stars are being born.

Sprinkled across
the image are also many globular and open clusters – groupings of
stars held together by their mutual gravity, as well as entire galaxies
beyond our own.

The two bright objects in the lower
right of the image are the Large and Small Magellanic Clouds, two dwarf
galaxies orbiting the Milky Way.

In small areas of the image where no
color information was available – to the lower left of the
Galactic center, to the upper left of the Small Magellanic Cloud, and
in the top portion of the map – an equivalent greyscale value was
assigned.

The second Gaia data release was
made public on 25 April 2018 and includes the position and brightness
of almost 1.7 billion stars, and the parallax, proper motion and color
of more than 1.3 billion stars. It also includes the radial velocity of
more than seven million stars, the surface temperature of more than 100
million stars, and the amount of dust intervening between us and of 87
million stars. There are also more than 500,000 variable sources, and
the position of 14,099 known Solar System objects – most of them
asteroids – included in the release.

Figure 63:
Gaia’s all-sky view of our Milky Way Galaxy and neighboring
galaxies. The maps show the total brightness and color of stars (top),
the total density of stars (middle) and the interstellar dust that
fills the Galaxy (bottom). These images are based on observations
performed by the ESA satellite in each portion of the sky between July
2014 and May 2016, which were published as part of Gaia second data
release on 25 April 2018 (image credit:ESA/Gaia/DPAC). 79)
Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC);
Top and middle: A. Moitinho / A. F. Silva / M. Barros / C. Barata,
University of Lisbon, Portugal; H. Savietto, Fork Research,
Portugal;Bottom: Gaia Coordination Unit 8; M. Fouesneau / C.
Bailer-Jones, Max Planck Institute for Astronomy, Heidelberg, Germany.

- “Gaia is an ambitious
mission that relies on a huge human collaboration to make sense of a
large volume of highly complex data. It demonstrates the need for
long-term projects to guarantee progress in space science and
technology and to implement even more daring scientific missions of the
coming decades.”

- Gaia was launched in December 2013 and started science operations the following year. The first data release, based on just over one year of observations, was published in 2016; it contained distances and motions of two million stars.

- The new data release, which covers
the period between 25 July 2014 and 23 May 2016, pins down the
positions of nearly 1.7 billion stars, and with a much greater
precision. For some of the brightest stars in the survey, the level of
precision equates to Earth-bound observers being able to spot a Euro
coin lying on the surface of the Moon.

- The new catalog lists the parallax
and velocity across the sky, or proper motion, for more than 1.3
billion stars. From the most accurate parallax measurements, about ten
per cent of the total, astronomers can directly estimate distances to
individual stars.

- “The second Gaia data
release represents a huge leap forward with respect to ESA’s
Hipparcos satellite, Gaia’s predecessor and the first space
mission for astrometry, which surveyed some 118 000 stars almost thirty
years ago,” says Anthony Brown of Leiden University, The
Netherlands. Anthony is the chair of the Gaia Data Processing and
Analysis Consortium Executive, overseeing the large collaboration of
about 450 scientists and software engineers entrusted with the task of
creating the Gaia catalogue from the satellite data.

- As well as positions, the data
include brightness information of all surveyed stars and color
measurements of nearly all, plus information on how the brightness and
color of half a million variable stars change over time. It also
contains the velocities along the line of sight of a subset of seven
million stars, the surface temperatures of about a hundred million and
the effect of interstellar dust on 87 million.

- Gaia also observes objects in our
Solar System: the second data release comprises the positions of more
than 14 000 known asteroids, which allows precise determination of
their orbits. A much larger asteroid sample will be compiled in
Gaia’s future releases.

- Further afield, Gaia closed in on
the positions of half a million distant quasars, bright galaxies
powered by the activity of the supermassive black holes at their cores.
These sources are used to define a reference frame for the celestial
coordinates of all objects in the Gaia catalog, something that is
routinely done in radio waves but now for the first time is also
available at optical wavelengths.

- Major discoveries are expected to
come once scientists start exploring Gaia’s new release. An
initial examination performed by the data consortium to validate the
quality of the catalog has already unveiled some promising surprises
– including new insights on the evolution of stars.

Figure 64: The second data
release from ESA’s Gaia mission contains a high-precision catalog
of the entire sky, covering celestial objects near and far. It includes
objects such as asteroids in our Solar System as well as the stellar
population of our Milky Way Galaxy and its satellites – globular
clusters and nearby galaxies. It also extends to distant quasars that
are being used to define a new celestial reference system. -This
infographic summarizes the cosmic scales covered by this comprehensive
dataset, which provides a wide range of topics for the astronomy
community (image credit: ESA, CC BY-SA 3.0 IGO) 80)

- “The new Gaia data are so
powerful that exciting results are just jumping at us,” says
Antonella Vallenari from the Istituto Nazionale di Astrofisica (INAF)
and the Astronomical Observatory of Padua, Italy, deputy chair of the
data processing consortium executive board.

- “For example, we have built
the most detailed Hertzsprung-Russell diagram of stars ever made on the
full sky and we can already spot some interesting trends. It feels like
we are inaugurating a new era of Galactic archeology.”

- Hertzsprung-Russell diagram: Named
after the two astronomers who devised it in the early twentieth
century, the Hertzsprung-Russell diagram compares the intrinsic
brightness of stars with their color and is a fundamental tool to study
populations of stars and their evolution.

- A new version of this diagram,
based on four million stars within five thousand light-years from the
Sun selected from the Gaia catalog, reveals many fine details for the
first time. This includes the signature of different types of white
dwarfs – the dead remnants of stars like our Sun – such
that a differentiation can be made between those with hydrogen-rich
cores and those dominated by helium.

- Combined with
Gaia measurements of star velocities, the diagram enables astronomers
to distinguish between various populations of stars of different ages
that are located in different regions of the Milky Way, such as the
disc and the halo, and that formed in different ways. Further scrutiny
suggests that the fast-moving stars thought to belong to the halo
encompass two stellar populations that originated via two different
formation scenarios, calling for more detailed investigations.

- “Gaia will greatly advance
our understanding of the Universe on all cosmic scales,” says
Timo Prusti, Gaia project scientist at ESA. “Even in the
neighborhood of the Sun, which is the region we thought we understood
best, Gaia is revealing new and exciting features.”

Figure 65:
More than four million stars within five thousand light-years from the
Sun are plotted on this diagram using information about their
brightness, color and distance from the second data release from
ESA’s Gaia satellite. It is known as a Hertzsprung-Russell
diagram after the astronomers who devised it in the early 20th century,
and it is a fundamental tool to study populations of stars and their
evolution (image credit: ESA/Gaia/DPAC) 81)
Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC);
Carine Babusiaux, IPAG – Université Grenoble Alpes, GEPI
– Observatoire de Paris, France.

Legend to Figure 65:
This Hertzsprung-Russell diagram, obtained by a selection of stars in
Gaia’s second release catalog, is the most detailed to date made
by mapping stars over the entire sky, containing roughly a hundred
times more stars than the one obtained using data from ESA’s
Hipparcos mission, the predecessor of Gaia, in the 1990s. This new
diagram contains so much highly accurate information that astronomers
have been able to identify fine details that were never before seen.

The Hertzsprung-Russell diagram can
be imagined as a stellar family portrait: stars are plotted according
to their color (on the horizontal axis) and brightness (on the vertical
axis) and are grouped in different regions of the diagram depending
mainly on their masses, chemical composition, ages, and stages in the
stellar life cycle. Information about stellar distances is fundamental
to calculate the true brightness, or absolute magnitude, of stars.

Brighter stars are shown in the top
part of the diagram, while fainter stars are in the lower part. Bluer
stars, which have hotter surfaces, are on the left, and redder stars,
with cooler surfaces, on the right. The color scale in this image does
not represent the color of stars but is a representation of how many
stars are plotted in each portion of the diagram: black represents
lower numbers of stars, while red, orange and yellow correspond to
increasingly higher numbers of stars.

The large diagonal stripe across the
center of the graph is known as the main sequence. This is where
fully-fledged stars that are generating energy by fusing hydrogen into
helium are found. Massive stars, which have bluer or whiter colors, are
found in the upper left end of the main sequence, while
intermediate-mass stars like our Sun, characterized by yellow colors,
are located mid-way. Redder, low-mass stars are found towards the lower
right.

As stars age they swell up, becoming
brighter and redder. Stars experiencing this are shown on the diagram
as the vertical arm leading off the main sequence and turning to the
right. This is known as the red giant branch.

While the most massive stars swell
into red giants and explode as powerful supernovae, stars like our Sun
end their days in a less spectacular fashion, eventually turning into
white dwarfs – the hot cores of dead stars. These are found in
the lower left of the diagram.

The huge leap forward from Hipparcos
to Gaia is especially visible in the white dwarf region of the diagram.
While Hipparcos had obtained reliable distance measurements to only a
handful of white dwarfs, more than 35,000 such objects are included in
this diagram based on Gaia data. This allows astronomers to see the
signature of different types of white dwarfs such that a
differentiation can be made between those with hydrogen-rich cores and
those dominated by helium.

- For a subset of stars within a few
thousand light-years of the Sun, Gaia has measured the velocity in all
three dimensions, revealing patterns in the motions of stars that are
orbiting the Galaxy at similar speeds.

- Future studies will confirm
whether these patterns are linked to perturbations produced by the
Galactic bar, a denser concentration of stars with an elongated shape
at the center of the Galaxy, by the spiral arm architecture of the
Milky Way, or by the interaction with smaller galaxies that merged with
it billions of years ago.

Legend to Figure 66:
This image combines the total amount of radiation detected by Gaia in
each pixel with measurements of the radiation taken through different
filters on the spacecraft to generate color information. Information
about the proper motion of stars – their velocity across the sky
– is represented as the texture of the image.

Measuring the proper motion of
several million stars in the LCM, astronomers were able to see an
imprint of the stars rotating clockwise around the center of the
galaxy. The image processing technique used to create this image is
called Line Integral Convolution.

• April 3, 2018: What is the first creature that comes to mind when you look at the dark cloud in this image of Figure 67?
Perhaps a dark kitten with a vivid white nose, front paws stretching
towards the right of the frame and tail up towards the left? Or perhaps
a fox, running with its mouth open and looking ahead, its vigilant eyes
pointing to the right? 83)84)

- In fact, this animal-themed shape
belongs to a dark nebula, a dense cloud of gas and dust in the
constellation of Orion, the Hunter, with the cat’s nose (or
fox’s eye) corresponding to the Orion Nebula Cluster, a star
cluster near the famous Orion Nebula, M42. The image is based on data from the first release of ESA’s Gaia satellite, and shows the density of stars observed while scanning that region of the sky.

- While this particular nebula is
not visible to the naked eye, similar clouds can be seen against the
bright background of the Milky Way from dark locations in the southern
hemisphere. Finding shapes in these dark nebulas is part of the
astronomical tradition of various cultures, from South America to
Australia, that include ‘dark cloud constellations’
resembling a variety of creatures in their firmaments.

- Launched in 2013, Gaia has been
charting more than a billion stars to unprecedented accuracy. This
information is extremely valuable to astronomers who are studying the
distribution of stars across our Galaxy.

- Even in the dark patches where
fewer stars are observed, Gaia’s meticulous census provides
important information to study the interstellar material that blocks
starlight. It is in these dark clouds of gas and dust that new
generations of stars come to life.

- The first data release
from Gaia, published in 2016, contained the position on the sky of more
than a billion stars, as well as the distance and motions of about two
million stars. Astronomers worldwide are now looking forward to the next data release,
planned for 25 April, which will include the distance and motions for
the full sample of stars, greatly extending the reach of the previous
survey.

- So far, Gaia data have been used
to study only the most nearby regions of star formation, within several
hundred light-years of us. With the new data, it will be possible to
investigate in great detail regions that are much farther away, like
the Orion star-forming complex, located some 1500 light-years from us,
and to estimate the 3D distribution not only of stars but also of the
dusty dark clouds where stars are born.

Figure 67:
Space Science Image of the Week: Some interstellar clouds take on
interesting shapes, like in this animal-themed Gaia image of the Orion
constellation (image credit: ESA/Gaia/DPAC)

• March 21, 2018: Last
month, ESA's Gaia satellite experienced a technical anomaly followed by
a 'safe mode' event. After thorough examination, the spacecraft was
successfully recovered and resumed normal scientific operations, while
the mission team keeps investigating the exact cause of the anomaly.85)

- On 18 February,
errors of two electrical units on the service module of Gaia led the
spacecraft to trigger an automatic safe mode. Safe modes occur when
certain spacecraft parameters fall out of their normal operating ranges
and the spacecraft automatically takes measures to preserve its safety.
During this safe mode, the science instruments were disabled in order
to protect them, and telecommunication with Earth took place through
the spacecraft's low-gain antenna.

- Following the anomaly, the mission
team conducted an initial inquiry into what caused the spacecraft to
activate the safe mode. They quickly identified the problem as deriving
from a failure in one of the two transponders on board Gaia, but the
root cause of the malfunction is still being investigated. After an
in-depth inquiry, the team recovered the satellite, which went back to
its normal scientific operations on 28 February using the second
identical back-up transponder.

- The team is still investigating
the origin of the anomaly and its possible relation to the lifetime of
the second transponder. Meanwhile, Gaia has been collecting data since
it resumed operations at the end of last month.

- Scientists worldwide are looking forward to the second data release
of Gaia, which will take place on 25 April and is based on observations
performed between mid 2014 and mid 2016. The mission has already
collected all data needed for its third release; these data will be processed and analyzed over the next few years.

• February 27, 2018: Last
year, ESA's Gaia mission helped astronomers make unique observations of
Neptune's largest moon, Triton, as it passed in front of a distant
star. This is a preview of the superb quality and versatility of the
Gaia data that will be released in April. 86)

- When a small Solar System body
such as a moon or an asteroid passes in front of a star and temporarily
blocks its light, the occultation is an extraordinary chance for
astronomers to study the properties of the foreground object. And, of
course, the more accurate the prediction of both objects' positions on
the sky, the better the observations.

- This is why, when a group of
astronomers were planning to observe the rare occultation of a distant
star by Neptune's moon Triton on 5 October 2017, they made a special request to the Gaia team.

- The astronomers, led by Bruno
Sicardy from Pierre and Marie Curie University and the Observatory of
Paris, France, had used all available observations to compute the path
that the moon's shadow would sweep across our planet. Within less than
three minutes, the occultation would first cross Europe and North
Africa, rapidly moving towards North America.

- They knew that somewhere, within
this couple of thousand kilometer-wide stretch, would lie a very
special thin strip, only about 100-km across. Observers situated on
this strip would be perfectly aligned with both Triton and the distant
star, and therefore able to see the so-called central flash.

- This sharp brightening of the star
happens half way through the occultation, and is caused by focussing of
the starlight by deep layers in the moon's atmosphere – about 10
km above surface. The central flash contains all-important information
to study the profile of Triton's atmosphere and the possible presence
of haze in it.

- To narrow down the best locations
to observe the occultation, and possibly the flash, the astronomers
turned to Gaia and its unprecedentedly accurate measurements of the
positions of more than a billion stars.

- They obtained a first estimate
using the star's position from the first batch of Gaia data (Gaia DR1),
which were publicly released in 2016. However, knowledge of the star's
proper motion – how it moves across the sky over the years
– would substantially improve their estimate.

- So they approached the Gaia team,
asking to receive extra information about the star's motion across the
sky from the upcoming second release of Gaia data (Gaia DR2), which is
planned for 25 April 2018.

- Having recognized the importance of these observations, the Gaia team published not only the preliminary position and proper motion of the occulted star from DR2, but also the positions of 453 other stars
that could be used to refine the estimate of Triton's orbit. With this
additional information, they computed again the location of the thin
strip where the central flash would be observed, shifting it roughly
300 km southwards of the earlier prediction.

- Come 5 October, a large
collaboration of professional as well as amateur astronomers scattered
across three continents were ready to observe Triton's occultation at
more than a hundred stations.

- Almost eighty of them were able to
monitor the phenomenon, and the improved prediction of observing
locations based on the specially released, preliminary data from Gaia
DR2 led to twenty-five successful detections of the central flash, from
Spain and Portugal to the south of France and the north of Italy. The
astronomers are now busy analyzing the data collected during this
campaign to learn more about the atmosphere of Triton.

- This occultation was a rare
opportunity to detect possible changes in Triton's atmospheric pressure
almost thirty years after the flyby of NASA's Voyager probe in August
1989. Observations of the central flash can also provide unique
information to detect possible winds near Triton's surface; current
analysis of the data indicates a quiet and still atmosphere.

- With the release of the position,
parallax and proper motion of more than 1.3 billion stars measured with
unprecedented accuracy, Gaia will provide an invaluable resource for
all branches of astrophysics. It will also be of great help to
professional and amateur astronomers who will be planning the
observation of stellar occultations by Solar System bodies in the
future, including that of another star by Triton on 6 October 2022.

- The star that
was occulted by Triton on 5 October 2017 is UCAC4 410-143659, a 12.7
V-magnitude (12.2 G-magnitude) star, situated in the constellation of
Aquarius. — Observations of this occultation by Triton are
coordinated by Bruno Sicardy (Université Pierre et Marie Curie
and Observatoire de Paris), leader of the ERC Lucky Star project.

• January 29, 2018: If you
gazed at the night sky over the past few weeks, it is possible that you
stumbled upon a very bright star near the Orion constellation. This is
Sirius, the brightest star of the entire night sky, which is visible
from almost everywhere on Earth except the northernmost regions. It is,
in fact, a binary stellar system, and one of the nearest to our Sun
– only eight light-years away. 87)

- Known since antiquity, this star
played a key role for the keeping of time and agriculture in Ancient
Egypt, as its return to the sky was linked to the annual flooding of
the Nile. In Ancient Greek mythology, it represented the eye of the
Canis Major constellation, the Great Dog that diligently follows Orion,
the Hunter.

- Dazzling stars like Sirius are
both a blessing and a curse for astronomers. Their bright appearance
provides plenty of light to study their properties, but also outshines
other celestial sources that happen to lie in the same patch of sky.

- This is why Sirius has been masked
in this picture taken by amateur astronomer Harald Kaiser on 10 January
from Karlsruhe, a city in the southwest of Germany.

- Once the glare of Sirius is removed, an interesting object becomes visible to its left: the stellar cluster Gaia 1, first spotted last year using data from ESA’s Gaia satellite.

- Gaia 1 is an open cluster –
a family of stars all born at the same time and held together by
gravity – and it is located some 15,000 light-years away. Its
chance alignment next to nearby, bright Sirius kept it hidden to
generations of astronomers that have been sweeping the heavens with
their telescopes over the past four centuries. But not to the
inquisitive eye of Gaia, which has been charting more than a billion
stars in our Milky Way galaxy.

- Mr. Kaiser heard about the
discovery of this cluster during a public talk on the Gaia mission and
zealously waited for a clear sky to try and image it using his 30 cm
diameter telescope. After covering Sirius on the telescope sensor
– creating the dark circle on the image – he succeeded at
recording some of the brightest stars of the Gaia 1 cluster.

- Gaia 1 is one of two previously unknown star clusters that have been discovered
by counting stars from the first set of Gaia data, which was released
in September 2016. Astronomers are now looking forward to Gaia’s second data release, planned for 25 April, which will provide vast possibilities for new, exciting discoveries.

- More information about opportunities for amateur astronomers to follow up on Gaia observations here.

Figure 70: Gaia 1 cluster image
taken from Karlsruhe (Germany) by Harald Kaiser using a 30 cm
telescope. The bright, central blob in the center of the image is
Sirius (image credit: Harald Kaiser)